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Abstract:

An M phase coupled inductor includes a magnetic core including a first
end magnetic element, a second end magnetic element, and M legs disposed
between and connecting the first and second end magnetic elements. M is
an integer greater than one. The coupled inductor further includes M
windings, where each winding has a substantially rectangular cross
section. Each one of the M windings is at least partially wound about a
respective leg.

Claims:

1. A coupled inductor, comprising: a ladder magnetic core including two
rails and M rungs, M being an integer greater than one; and M windings
having rectangular cross section, each of the M windings wound around a
respective one of the M rungs such that a long side of the rectangular
cross section is adjacent to the rung.

2. The coupled inductor of claim 1, M being greater than two.

3. The coupled inductor of claim 1, each of the M windings having two
ends, each end forming a respective solder tab for surface mount
attachment to a printed circuit board.

4. The coupled inductor of claim 1, each of the M windings forming a
single turn.

5. The coupled inductor of claim 1, each of the M windings forming a
plurality of turns.

6. The coupled inductor of claim 1, each of the M windings having a cross
sectional area with an aspect ratio of at least five.

7. A coupled inductor, comprising: a magnetic core, including: first and
second end magnetic elements separated by a linear separation distance,
and M legs each connected to the first and second end magnetic elements,
M being an integer greater than one; and M windings having rectangular
cross section, each of the M windings wound around a respective one of
the M legs such that a long axis of the winding's rectangular cross
section is parallel to the linear separation distance.

8. The coupled inductor of claim 7, M being greater than two.

9. The coupled inductor of claim 7, each of the M windings having two
ends, each end forming a respective solder tab for surface mount
attachment to a printed circuit board.

10. The coupled inductor of claim 7, each of the M windings forming a
single turn.

11. The coupled inductor of claim 7, each of the M windings forming a
plurality of turns.

12. The coupled inductor of claim 7, each of the M windings having a
cross sectional area with an aspect ratio of at least five.

13. A coupled inductor, comprising: a magnetic core; and first and second
windings each having substantially square cross section and wound around
a common leg of the magnetic core and through a passageway formed by the
magnetic core, the first and second windings separated by a linear
separation distance throughout the passageway.

14. The coupled inductor of claim 13, the passageway having depth and
height, the depth being greater than the height, the linear separation
distance being along an axis perpendicular to an axis of the depth and
perpendicular to an axis of the height, the separation distance greater
than the height.

15. A coupled inductor, comprising: a magnetic core, including: first and
second end magnetic elements, and M legs each connected to the first and
second end magnetic elements, M being an integer greater than one; and M
windings each having substantially square cross section, each of the M
windings wound around a respective one of the M legs.

16. The coupled inductor of claim 15, wherein: M is two; the first and
second end magnetic elements are separated by a first linear separation
distance; the M windings comprise a first and a second winding separated
by a second linear separation distance parallel to the first linear
separation distance in a passageway formed by the magnetic core.

17. The coupled inductor of claim 15, each of the M legs forming at least
one turn.

18. The coupled inductor of claim 15, M being greater than two.

19. The coupled inductor of claim 15, further comprising a printed
circuit board separating the M legs from the first and second end
magnetic elements.

20. The coupled inductor of claim 15, further comprising a printed
circuit board, the M legs extending through the printed circuit board.

Description:

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation-in-part of copending U.S. patent
application Ser. No. 12/271,497, filed 14 Nov. 2008, which is a
continuation-in-part of U.S. patent application Ser. No. 11/929,827,
filed 30 Oct. 2007, now U.S. Pat. No. 7,498,920, which is a
continuation-in-part of U.S. patent application Ser. No. 11/852,207,
filed 7 Sep. 2007, which is a divisional of U.S. patent application Ser.
No. 10/318,896, filed 13 Dec. 2002, now U.S. Pat. No. 7,352,269. U.S.
patent application Ser. No. 12/271,497 is also a continuation of
International Patent Application No. PCT/US08/81886, filed 30 Oct. 2008,
which claims benefit of priority to U.S. patent application Ser. No.
11/929,827, filed 30 Oct. 2007 and to U.S. Provisional Patent Application
Ser. No. 61/036,836 filed 14 Mar. 2008. U.S. patent application Ser. No.
12/271,497 also claims benefit of priority to U.S. Provisional Patent
Application Ser. No. 61/036,836, filed 14 Mar. 2008. All of the
above-mentioned applications are incorporated herein by reference.

BACKGROUND

[0002] A DC-to-DC converter, as known in the art, provides an output
voltage that is a step-up, a step-down, or a polarity reversal of the
input voltage source. Certain known DC-to-DC converters have parallel
power units with inputs coupled to a common DC voltage source and outputs
coupled to a load, such as a microprocessor. Multiple power-units can
sometimes reduce cost by lowering the power and size rating of
components. A further benefit is that multiple power units provide
smaller per-power-unit peak current levels, combined with smaller passive
components.

[0003] The prior art also includes switching techniques in
parallel-power-unit DC-to-DC converters. By way of example, power units
may be switched with pulse width modulation (PWM) or with pulse frequency
modulation (PFM). Typically, in a parallel-unit buck converter, the
energizing and de-energizing of the inductance in each power unit occurs
out of phase with switches coupled to the input, inductor and ground.
Additional performance benefits are provided when the switches of one
power unit, coupling the inductors to the DC input voltage or to ground,
are out of phase with respect to the switches in another power unit. Such
a "multi-phase," parallel power unit technique results in ripple current
cancellation at a capacitor, to which all the inductors are coupled at
their respective output terminals.

[0004] It is clear that smaller inductances are needed in DC-to-DC
converters to support the response time required in load transients and
without prohibitively costly output capacitance. More particularly, the
capacitance requirements for systems with fast loads, and large
inductors, may make it impossible to provide adequate capacitance
configurations, in part due to the parasitic inductance generated by a
large physical layout. But smaller inductors create other issues, such as
the higher frequencies used in bounding the AC peak-to-peak current
ripple within each power unit. Higher frequencies and smaller inductances
enable shrinking of part size and weight. However, higher switching
frequencies result in more heat dissipation and lower efficiency. In
short, small inductance is good for transient response, but large
inductance is good for AC current ripple reduction and efficiency.

[0005] The prior art has sought to reduce the current ripple in multiphase
switching topologies by coupling inductors. For example, one system set
forth in U.S. Pat. No. 5,204,809, incorporated herein by reference,
couples two inductors in a dual-phase system driven by an H bridge to
help reduce ripple current. In one article, Investigating Coupling
Inductors in the Interleaving QSW VRM, IEEE APEC (Wong, February 2000),
slight benefit is shown in ripple reduction by coupling two windings
using presently available magnetic core shapes. However, the benefit from
this method is limited in that it only offers slight reduction in ripple
at some duty cycles for limited amounts of coupling.

[0006] One known DC-to-DC converter offers improved ripple reduction that
either reduces or eliminates the afore-mentioned difficulties. Such a
DC-to-DC converter is described in commonly owned U.S. Pat. No. 6,362,986
issued to Schultz et al. ("the '986 patent"), incorporated herein by
reference. The '986 patent can improve converter efficiency and reduce
the cost of manufacturing DC-to-DC converters.

[0007] Specifically, the '986 patent shows one system that reduces the
ripple of the inductor current in a two-phase coupled inductor within a
DC-to-DC buck converter. The '986 patent also provides a multi-phase
transformer model to illustrate the working principles of multi-phase
coupled inductors. It is a continuing problem to address scalability and
implementation issues of DC-to-DC converters.

[0008] As circuit components and, thus, printed circuit boards (PCB),
become smaller due to technology advancements, smaller and more scalable
DC-to-DC converters are needed to provide for a variety of voltage
conversion needs.

SUMMARY

[0009] As used herein, a "coupled" inductor implies an interaction between
multiple inductors of different phases. Coupled inductors described
herein may be used within DC-to-DC converters or within a power converter
for power conversion applications, for example.

[0010] In an embodiment, an M phase coupled inductor includes a magnetic
core including a first end magnetic element, a second end magnetic
element, and M legs disposed between and connecting the first and second
end magnetic elements. M is an integer greater than one. Each leg has a
respective width in a direction connecting the first and second end
magnetic elements. The coupled inductor further includes M windings,
where each one of the M windings is at least partially wound about a
respective leg. Each winding has a substantially rectangular cross
section and a respective width that is at least eighty percent of the
width of its respective leg.

[0011] In an embodiment, an M phase coupled inductor includes a magnetic
core including a first end magnetic element, a second end magnetic
element, and M legs disposed between and connecting the first and second
end magnetic elements. M is an integer greater than one, and each leg has
an outer surface. The coupled inductor further includes M windings, where
each winding has a substantially rectangular cross section. Each one of
the M windings is at least partially wound about a respective leg such
that the winding diagonally crosses at least a portion of its leg's outer
surface.

[0012] In an embodiment, an M phase coupled inductor includes a magnetic
core including a first end magnetic element, a second end magnetic
element, and M legs disposed between and connecting the first and second
end magnetic elements. M is an integer greater than one, and each leg
forms at least two turns. The coupled inductor further includes M
windings, where each winding has a substantially rectangular cross
section. Each one of the M windings is at least partially wound about a
respective leg.

[0013] In an embodiment, an M phase coupled inductor includes a magnetic
core including a first end magnetic element, a second end magnetic
element, and M legs disposed between and connecting the first and second
end magnetic elements. M is an integer greater than two. The magnetic
core further includes M windings, where each winding has a substantially
rectangular cross section with an aspect ratio of at least two. Each one
of the M windings is at least partially wound about a respective leg.

[0014] In an embodiment, a multi-phase DC-to-DC converter includes an
M-phase coupled inductor and M switching subsystems. M is an integer
greater than two. The coupled inductor includes a magnetic core including
a first end magnetic element, a second end magnetic element, and M legs
disposed between and connecting the first and second end magnetic
elements. The coupled inductor further includes M windings, where each
winding has a substantially rectangular cross section, a first end, and a
second end. Each one of the M windings is at least partially wound about
a respective leg. Each switching subsystem is coupled to the first end of
a respective winding, and each switching subsystem switches the first end
of its respective winding between two voltages. Each second end is
electrically coupled together.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 shows one multi-phase DC-to-DC converter system, according
to an embodiment.

[0032]FIG. 18 is a side perspective view of one multi-phase coupled
inductor, according to an embodiment.

[0033]FIG. 19 is a top plan view of the multi-phase coupled inductor of
FIG. 18.

[0034]FIG. 20 is a top plan view of a two-phase embodiment of the coupled
inductor of FIGS. 18 and 19.

[0035]FIG. 21 is a side perspective view of one multi-phase coupled
inductor, according to an embodiment.

[0036]FIG. 22 is a top plan view of one inductor winding, according to an
embodiment.

[0037]FIG. 23 is a top perspective view of one embodiment of the winding
of FIG. 22.

[0038]FIG. 24 is a top perspective view of one M-phase coupled inductor,
according to an embodiment.

[0039]FIG. 25 is a top perspective view of one embodiment of the coupled
inductor of FIG. 24.

[0040] FIG. 26 is a side perspective view of one winding that may be used
with the coupled inductor of FIG. 24, according to an embodiment.

[0041] FIG. 27 is a side plan view of one leg of the coupled inductor of
FIG. 24 having an embodiment of the winding of FIG. 26, according to an
embodiment.

[0042]FIG. 28 is a bottom perspective view of an embodiment of the
winding of FIG. 26.

[0043]FIG. 29 is a top perspective view of another embodiment of the
coupled inductor of FIG. 24.

[0044]FIG. 30 is a top plan view of another embodiment of the coupled
inductor of FIG. 24.

[0045]FIG. 31 is a plan view of one side of the coupled inductor of FIG.
30.

[0046]FIG. 32 is a plan view of another side of the coupled inductor of
FIG. 30.

[0047] FIG. 33 is a top plan view of one PCB layout, according to an
embodiment.

[0048]FIG. 34 is a side perspective view of another winding that may be
used with the coupled inductor of FIG. 24, according to an embodiment.

[0049]FIG. 35 is a top plan view of an embodiment of the winding of FIG.
34 before being wound about a leg of a magnetic core.

[0050]FIG. 36 shows another embodiment of the coupled inductor of FIG. 24
disposed above solder pads, according to an embodiment.

[0051] FIG. 37 is a top plan view of one PCB layout, according to an
embodiment.

[0052]FIG. 38 is a side perspective view of another winding that may be
used with the coupled inductor of FIG. 24, according to an embodiment.

[0053]FIG. 39 is a top plan view of an embodiment of the winding of FIG.
38 before being wound about a leg of a magnetic core.

[0054]FIG. 40 shows another embodiment of the coupled inductor of FIG. 24
disposed above solder pads, according to an embodiment.

[0055] FIG. 41 is a top plan view of one PCB layout, according to an
embodiment.

[0056]FIG. 42 is a side perspective view of another winding that may be
used with the coupled inductor of FIG. 24, according to an embodiment.

[0057]FIG. 43 shows another embodiment of the coupled inductor of FIG. 24
disposed above solder pads, according to an embodiment.

[0058]FIG. 44 is a top plan view of one M-phase coupled inductor,
according to an embodiment.

[0059]FIG. 45 is a bottom perspective view of an embodiment of a winding
of the coupled inductor of FIG. 44 before being wound about a leg of the
coupled inductor.

[0060]FIG. 46 is a top plan view of one PCB layout, according to an
embodiment.

[0061]FIG. 47 is a top plan view of one M-phase coupled inductor,
according to an embodiment.

[0062]FIG. 48 is a bottom perspective view of a winding of the coupled
inductor of FIG. 47 before being wound about a leg of the coupled
inductor.

[0063]FIG. 49 is a side perspective view of one embodiment of the winding
of FIG. 48.

[0064]FIG. 50 is a top plan view of one embodiment of the coupled
inductor of FIG. 47.

[0065]FIG. 51 is a top plan view of one PCB layout, according to an
embodiment.

[0066]FIG. 52 is a top plan view of one magnetic core, according to an
embodiment.

[0067]FIG. 53 is an exploded top plan view of the magnetic core of FIG.
52.

[0068] FIG. 54 is a top plan view of one embodiment of the magnetic core
of FIG. 52.

[0069]FIG. 55 is an exploded top plan view of the magnetic core of FIG.
54.

[0070]FIG. 56 schematically illustrates one multiphase DC-to-DC
converter, according to an embodiment.

[0071]FIG. 57 is a perspective view of a coupled inductor including
windings having square cross section, according to an embodiment.

[0072]FIG. 58 shows a cross section of one of the windings of the FIG. 57
coupled inductor.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0073] It is noted that, for purposes of illustrative clarity, certain
elements in the drawings may not be drawn to scale. Specific instances of
an item may be referred to by use of a numeral in parentheses (e.g.,
winding 506(1)) while numerals without parentheses refer to any such item
(e.g., windings 506).

[0074] Embodiments of methods disclosed herein provide for constructing a
magnetic core. Such a core is, for example, useful in applications
detailed in the '986 patent. In one embodiment, the method provides for
constructing M-phase coupled inductors as both single and scalable
magnetic structures, where M is greater than 1. Some embodiments of
M-phase inductors described herein may include M-number of windings. One
embodiment of a method additionally describes construction of a magnetic
core that enhances the benefits of using the scalable M-phase coupled
inductor.

[0075] In one embodiment, the M-phase coupled inductor is formed by
coupling first and second magnetic cores in such a way that a planar
surface of the first core is substantially aligned with a planar surface
of the second core in a common plane. The first and second magnetic cores
may be formed into shapes that, when coupled together, may form a single
scalable magnetic core having desirable characteristics, such as ripple
current reduction and ease of implementation. In one example, the cores
are fashioned into shapes, such as a U-shape, an I-shape (e.g., a bar),
an H-shape, a ring-shape, a rectangular-shape, or a comb. In another
example, the cores could be fashioned into a printed circuit trace within
a PCB.

[0076] In some embodiments, certain cores form passageways through which
conductive windings are wound when coupled together. Other cores may
already form these passageways (e.g., the ring-shaped core and the
rectangularly shaped core). For example, two H-shaped magnetic cores may
be coupled at the legs of each magnetic core to form a passageway. As
another example, a multi-leg core may be formed as a comb-shaped core
coupled to an I-shaped core. In yet another example, two I-shaped cores
are layered about a PCB such that passageways are formed when the two
cores are coupled to one another at two or more places, or when
pre-configured holes in the PCB are filled with a ferromagnetic powder.

[0077] Advantages of some embodiments of methods and structures disclosed
herein include a scalable and cost effective DC-to-DC converters that
reduce or nearly eliminate ripple current. The methods and structures of
some embodiments further techniques that achieve the benefit of various
performance characteristics with a single, scalable, topology.

[0078]FIG. 1 shows a multi-phase DC-to-DC converter system 10. System 10
includes a power source 12 electrically coupled with M switches 14 and M
inductors 24, with M≧2, for supplying power to a load 16. Each
switch and inductor pair 14, 24 represent one phase 26 of system 10, as
shown. Inductors 24 cooperate together as a coupled inductor 28. Each
inductor 24 has, for example, a leakage inductance value ranging from 10
nanohenrys ("nH") to 200 nH; such exemplary leakage inductance values may
enable system 10 to advantageously have a relatively low ripple voltage
magnitude and an acceptable transient response at a typical switching
frequency. Power source 12 may, for example, be either a DC power source,
such as a battery, or an AC power source cooperatively coupled to a
rectifier, such as a bridge rectifier, to provide DC power in signal 18.
Each switch 14 may include a plurality of switches to perform the
functions of DC-to-DC converter system 10.

[0079] In operation, DC-to-DC converter system 10 converts an input signal
18 from source 12 to an output signal 30. The voltage of signal 30 may be
controlled through operation of switches 14, to be equal to or different
from signal 18. Specifically, coupled inductor 28 has one or more
windings (not shown) that extend through and about inductors 24, as
described in detail below. These windings attach to switches 14, which
collectively operate to regulate the output voltage of signal 30 by
sequentially switching inductors 24 to signal 18.

[0080] When M=2, system 10 may for example be used as a two-phase power
converter (e.g., power supply). System 10 may also be used in both DC and
AC based power supplies to replace a plurality of individual discrete
inductors such that coupled inductor 28 reduces inductor ripple current,
filter capacitances, and/or PCB footprint sizes, while delivering higher
system efficiency and enhanced system reliability. Other functional and
operational aspects of DC-to-DC converter system 10 may be exemplarily
described in the '986 patent. Some embodiments of coupled inductor 28 are
described as follows.

[0081] Those skilled in the art should appreciate that system 10 may be
arranged with different topologies to provide a coupled inductor 28 and
without departing from the scope hereof. For example, in another
embodiment of system 10, a first terminal 8 of each inductor 24 is
electrically coupled together and directly to source 12. In such
embodiment, a respective switch 14 couples second terminal 9 of each
inductor 24 to load 16. As another example, although each inductor 24 is
illustrated in FIG. 1 as being part of coupled inductor 28, one or more
of inductors 24 may be discrete (non-coupled) inductors. Additionally,
single coupled inductor 28 illustrated in FIG. 1 may be replaced with a
plurality of coupled inductors 28. For example, an embodiment of system
10 having six phases may include a quantity of three two-phase coupled
inductors. Furthermore, some embodiments of system 10 include one or more
transformers to provide electrical isolation.

[0082]FIG. 2 shows a two-phase coupled inductor 33, in accord with one
embodiment. Inductor 33 may, for example, serve as inductor 28 of FIG. 1,
with M=2. The two-phase coupled inductor 33 may include a first magnetic
core 36A and a second magnetic core 36B. The first and second magnetic
cores 36A, 36B, respectively, are coupled together such that planar
surfaces 37A, 37B, respectively, of each core are substantially aligned
in a common plane, represented by line 35. When the two magnetic cores
36A and 36B are coupled together, they cooperatively form a single
magnetic core for use as a two-phase coupled inductor 33.

[0083] In this embodiment, the first magnetic core 36A may be formed from
a ferromagnetic material into a U-shape. The second magnetic core 36B may
be formed from the same ferromagnetic material into a bar, or I-shape, as
shown. As the two magnetic cores 36A, 36B are coupled together, they form
a passageway 38 through which windings 34A, 34B are wound. The windings
34A, 34B may be formed of a conductive material, such as copper, that
wind though and about the passageway 38 and the magnetic core 36B.
Moreover, those skilled in the art should appreciate that windings 34A,
34B may include a same or differing number of turns about the magnetic
core 36B. Windings 34A, 34B are shown as single turn windings, to
decrease resistance through inductor 33.

[0084] The windings 34A and 34B of inductor 33 may be wound in the same or
different orientation from one another. The windings 34A and 34B may also
be either wound about the single magnetic core in the same number of
turns or in a different number of turns. The number of turns and
orientation of each winding may be selected so as to support the
functionality of the '986 patent, for example. By orienting the windings
34A and 34B in the same direction, the coupling is directed so as to
reduce the ripple current flowing in windings 34A, 34B.

[0085] Those skilled in the art should appreciate that a gap (not shown)
may exist between magnetic cores 36A, 36B, for example to reduce the
sensitivity to direct current, when inductor 33 is used within a
switching power converter. Such a gap is for example illustratively
discussed as dimension A, FIG. 5.

[0086] The dimensional distance between windings 34A, 34B may also be
adjusted to adjust leakage inductance. Such a dimension is illustratively
discussed as dimension E, FIG. 5.

[0087] As shown, magnetic core 36A is a "U-shaped" core while magnetic
core 36B is an unshaped flat plate. Those skilled in the art should also
appreciate that coupled inductor 33 may be formed with magnetic cores
with different shapes. By way of example, two "L-shaped" or two
"U-shaped" cores may be coupled together to provide like overall form as
combined cores 36A, 36B, to provide like functionality within a switching
power converter. Cores 36A, 36B may be similarly replaced with a solid
magnetic core block with a hole therein to form passageway 38. At least
part of passageway 38 is free from intervening magnetic structure between
windings 34A, 34B; air or non-magnetic structure may for example fill the
space of passageway 38 and between the windings 34A, 34B. In one
embodiment, intervening magnetic structure fills no more than 50% of a
cross-sectional area between windings 34A, 34B, and within passageway 38;
by way of example, the cross-sectional area of passageway 38 may be
defined by the plane of dimensions 39A (depth), 39B (height), which is
perpendicular to a line 39C (separation distance) between windings 34A,
34B.

[0088]FIG. 2 also illustrates one advantageous feature associated with
windings 34A, 34B. Specifically, each of windings 34A, 34B is shown with
a rectangular cross-section that, when folded underneath core 36B, as
shown, produces a tab for soldering to a PCB, and without the need for a
separate item. Other windings discussed below may have similar beneficial
features.

[0090]FIG. 3 shows a single two-phase ring-core coupled inductor 43, in
accord with one embodiment. Inductor 43 may be combined with other
embodiments herein, for example, to serve as inductor 28 of FIG. 1. The
ring-core inductor 43 is formed from a ring magnetic core 44. The core 44
has a passageway 45; windings 40 and 42 are wound through passageway 45
and about the core 44, as shown. In this embodiment, core 44 is formed as
a single magnetic core; however multiple magnetic cores, such as two
semi-circles, may be cooperatively combined to form a similar core
structure. Other single magnetic core embodiments shown herein may also
be formed by cooperatively combining multiple magnetic cores as discussed
in FIG. 17. Such a combination may align plane 44P of magnetic core 44 in
the same plane of other magnetic cores 44, for example to facilitate
mounting to a PCB. At least part of passageway 45 is free from
intervening magnetic structure between windings 40, 42; air may for
example fill the space of passageway 45 and between windings 40, 42. In
one embodiment, intervening magnetic structure fills no more than 50% of
a cross-sectional area between windings 40, 42, and within passageway 45.

[0091] In one embodiment, windings 40, 42 wind through passageway 45 and
around ring magnetic core 44 such that ring magnetic core 44 and windings
40, 42 cooperate with two phase coupling within a switching power
converter. Winding 40 is oriented such that DC current in winding 40
flows in a first direction within passageway 45; winding 42 is oriented
such that DC current in winding 42 flows in a second direction within
passageway 45, where the first direction is opposite to the second
direction. Such a configuration avoids DC saturation of core 44, and
effectively reduces ripple current. See U.S. Pat. No. 6,362,986.

[0092]FIG. 4 shows a vertically mounted two-phase coupled inductor 54, in
accord with one embodiment. Inductor 54 may be combined and/or formed
with other embodiments herein, for example, to serve as inductor 28 of
FIG. 1. The inductor 54 is formed as a rectangular-shaped magnetic core
55. The core 55 forms a passageway 56; windings 50 and 52 may be wound
through passageway 56 and about the core 55. In this embodiment, the
inductor 54 may be vertically mounted on a plane of PCB 57 (e.g., one end
of passageway 56 faces the plane of the PCB 57) so as to minimize a
"footprint", or real estate, occupied by the inductor 54 on the PCB 57.
This embodiment may improve board layout convenience. Windings 50 and 52
may connect to printed traces 59A, 59B on the PCB 57 for receiving
current. Additionally, windings 50 and 52 may be used to mount inductor
54 to the PCB 57, such as by flat portions 50P, 52P of respective
windings 50, 52. Specifically, portions 50P, 52P may be soldered
underneath to PCB 57. At least part of passageway 56 is free from
intervening magnetic structure between windings 50, 52; air may for
example fill the space of passageway 56 and between windings 50, 52. In
one embodiment, intervening magnetic structure fills no more than 50% of
a cross-sectional area between windings 50, 52, and within passageway 56;
by way of example, the cross-sectional area of passageway 56 may be
defined by the plane of dimensions 53A (height), 53B (depth), which is
perpendicular to a line 53C (separation distance) between windings 50,
52. Also shown in FIG. 4 are widths 352 and 354, legs 356 and 358,
surfaces 360, 362, 364, 366, 368, 372, and 374.

[0093]FIG. 4 further has advantages in that one winding 50 winds around
one side of core 55, while winding 52 winds around another side of core
55, as shown. Such a configuration thus provides for input on one side of
inductor 54 and output on the other side with convenient mating to a
board layout of PCB 57.

[0094]FIG. 5 shows a two-phase coupled inductor 60, in accord with one
embodiment. Inductor 60 may, for example, serve as inductor 28 of FIG. 1.
The inductor 60 may be formed from first and second magnetic cores 61 and
62, respectively. The illustration of the cores 61 and 62 is exaggerated
for the purpose of showing detail of inductor 60. The two cores 61 and 62
may be "sandwiched" about the windings 64 and 63. The dimensions E, C and
A, in this embodiment, are part of the calculation that determines a
leakage inductance for inductor 60. The dimensions of D, C, and A,
combined with the thickness of the first and second cores 61 and 62, are
part of the calculation that determines a magnetizing inductance of the
inductor 60. For example, assuming dimension D is much greater than E,
the equations for leakage inductance and magnetizing inductance can be
approximated as:

where μ0 is the permeability of free space, Ll is leakage
inductance, and Lm is magnetizing inductance. One advantage of this
embodiment is apparent in the ability to vary the leakage and the
magnetizing inductances by varying the dimensions of inductor 60. For
example, the leakage inductance and the magnetizing inductance can be
controllably varied by varying the dimension E (e.g., the distance
between the windings 64 and 63). In one embodiment, the cores 61 and 62
may be formed as conductive prints, or traces, directly with a PCB,
thereby simplifying assembly processes of circuit construction such that
windings 63, 64 are also PCB traces that couple through one or more
planes of a multi-plane PCB. In one embodiment, the two-phase inductor 60
may be implemented on a PCB as two parallel thin-film magnetic cores 61
and 62. In another embodiment, inductor 60 may form planar surfaces 63P
and 64P of respective windings 63, 64 to facilitate mounting of inductor
60 onto the PCB. Dimensions E, A between windings 63, 64 may define a
passageway through inductor 60. At least part of this passageway is free
from intervening magnetic structure between windings 63, 64; air may for
example fill the space of the passageway and between windings 63, 64. In
one embodiment, intervening magnetic structure fills no more than 50% of
a cross-sectional area between windings 63, 64, and within the
passageway; by way of example, the cross-sectional area of the passageway
may be defined by the plane of dimensions A, C, which is perpendicular to
a line parallel to dimension E between windings 63, 64.

[0095]FIG. 6 shows a scalable, multi-phase coupled inductor 70 that may
be formed from a plurality of H-shaped magnetic cores 74, in accord with
one embodiment. Inductor 70 may, for example, serve as inductor 28 of
FIG. 1. The inductor 70 may be formed by coupling "legs" 74A of each
H-shaped core 74 together. Each core 74 has one winding 72. The windings
72 may be wound through the passageways 71 formed by legs 74A of each
core 74. The winding of each core 74 may be wound prior to coupling the
several cores together such that manufacturing of inductor 70 is
simplified. By way of example, cores 74 may be made and used later; if a
design requires additional phases, more of the cores 74 may be coupled
together "as needed" without having to form additional windings 72. Each
core 74 may be mounted on a PCB, such as PCB 57 of FIG. 4, and be coupled
together to implement a particular design. One advantage to inductor 70
is that a plurality of cores 74 may be coupled together to make a
multi-core inductor that is scalable. In one embodiment, H-shaped cores
74 cooperatively form a four-phase coupled inductor. Other embodiments
may, for example, scale the number of phases of the inductor 70 by
coupling more H-shaped cores 74. For example, the coupling of another
H-shaped core 74 may increase the number of phases of the inductor 70 to
five. In one embodiment, the center posts 74C about which the windings 72
are wound may be thinner (along direction D) than the legs 74A (along
direction D). Thinner center posts 74C may reduce winding resistance and
increase leakage inductance without increasing the footprint size of the
coupled inductor 70. Each of the H-shaped cores 74 has a planar surface
74P, for example, that aligns with other H-shaped cores in the same plane
and facilitates mounting of inductor 70 onto PCB 74S. At least part of
one passageway 71, at any location along direction D within the one
passageway, is free from intervening magnetic structure between windings
72; for example air may fill the three central passageways 71 of inductor
70 and between windings 72 in those three central passageways 71. In one
embodiment, intervening magnetic structure fills no more than 50% of a
cross-sectional area between windings 72, and within passageway 71.

[0096]FIG. 7 shows a scalable, multi-phase coupled inductor 75 formed
from a plurality of U-shaped magnetic cores 78 and an equal number of
I-shaped magnetic cores 79 (e.g., bars), in accord with one embodiment.
Inductor 75 may, for example, serve as inductor 28 of FIG. 1. The
U-shaped cores 78 coupled with the I-shaped cores 79 may form
rectangular-shaped core cells 75A, 75B, 75C, and 75D, each of which is
similar to the cell of FIG. 2, but for the winding placement. The
inductor 75 may be formed by coupling each of the rectangular-shaped core
cells 75A, 75B, 75C, and 75D together. The windings 76 and 77 may be
wound through the passageways (labeled "APERTURE") formed by the
couplings of cores 78 with cores 79 and about core elements. Similar to
FIG. 6, the windings 76 and 77 of each rectangular-shaped core cell may
be made prior to coupling with other rectangular-shaped core cells 75A,
75B, 75C, and 75D such that manufacturing of inductor 75 is simplified;
additional inductors 75, may thus, be implemented "as needed" in a
design. One advantage to inductor 75 is that cells 75A, 75B, 75C, and
75D--and/or other like cells--may be coupled together to make inductor 75
scalable. In the illustrated embodiment of FIG. 7, rectangular-shaped
cells 75A, 75B, 75C, and 75D cooperatively form a five-phase coupled
inductor. Each of the I-shaped cores 79 has a planar surface 79P, for
example, that aligns with other I-shaped cores in the same plane and
facilitates mounting of inductor 75 onto PCB 79S. At least part of the
Apertures is free from intervening magnetic structure between windings
76, 77; air may for example fill the space of these passageways and
between windings 76, 77. By way of example, each Aperture is shown with a
pair of windings 76, 77 passing therethrough, with only air filling the
space between the windings 76, 77. In one embodiment, intervening
magnetic structure fills no more than 50% of a cross-sectional area
between windings 76, 77, and within each respective Aperture.

[0097] FIG. 8 shows a scalable, multi-phase coupled inductor 80 formed
from a plurality of U-shaped magnetic cores 81 (or C-shaped depending on
the orientation), in accord with one embodiment. Each magnetic core 81
has two lateral members 81L and an upright member 81U, as shown. Inductor
80 may, for example, serve as inductor 28 of FIG. 1. The inductor 80 may
be formed by coupling lateral members 81L of each U-shaped core 81
(except for the last core 81 in a row) together with the upright member
81U of a succeeding U-shaped core 81, as shown. The windings 82 and 83
may be wound through the passageways 84 formed between each pair of cores
81. Scalability and ease of manufacturing advantages are similar to those
previously mentioned. For example, winding 82 and its respective core 81
may be identical to winding 83 and its respective core 81, forming a pair
of like cells. More cells can be added to desired scalability. Each of
the U-shaped cores 81 has a planar surface 81P, for example, that aligns
with other U-shaped cores 81 in the same plane and facilitates mounting
of inductor 80 onto PCB 81S. At least part of one passageway 84 is free
from intervening magnetic structure between windings 82, 83; air may for
example fill the space of this passageway 84 and between windings 82, 83.
By way of example, three passageways 84 are shown each with a pair of
windings 82, 83 passing therethrough, with only air filling the space
between the windings 82, 83. One winding 82 is at the end of inductor 80
and does not pass through such a passageway 84; and another winding 83 is
at another end of inductor 80 and does not pass through such a passageway
84. In one embodiment, intervening magnetic structure fills no more than
50% of a cross-sectional area between windings 82, 83, and within
passageway 84.

[0098]FIG. 9 shows a multi-phase coupled inductor 85 formed from a
comb-shaped magnetic core 86 and an I-shaped (e.g., a bar) magnetic core
87, in accord with one embodiment. Inductor 85 may, for example, serve as
inductor 28 of FIG. 1. The inductor 85 may be formed by coupling a planar
surface 86P of "teeth" 86A of the comb-shaped core 86 to a planar surface
87P of the I-shaped core 87 in substantially the same plane. The windings
88 and 89 may be wound through the passageways 86B formed by adjacent
teeth 86A of comb-shaped core 86 as coupled with I-shaped core 87. The
windings 88 and 89 may be wound about the teeth 86A of the comb-shaped
core 86. FIG. 9 also shows end passageways 200, surfaces 202, 204, 206,
208, 210, 212, 214, and 224, height 216, depth 218, and widths 220 and
222. This embodiment may also be scalable by coupling inductor 85 with
other inductor structures shown herein. For example, the U-shaped
magnetic cores 81 of FIG. 8 may be coupled to inductor 85 to form a
multi-phase inductor, or a M+1 phase inductor. The I-shaped core 87 has a
planar surface 87P, for example, that facilitates mounting of inductor 85
onto PCB 87S. At least part of one passageway 86B is free from
intervening magnetic structure between windings 88, 89; air may for
example fill the space of this passageway 86B and between windings 88,
89. By way of example, three passageways 86B are shown each with a pair
of windings 88, 89 passing therethrough, with only air filling the space
between the windings 88, 89. One winding 88 is at the end of inductor 85
and does not pass through such a passageway 86B; and another winding 89
is at another end of inductor 85 and does not pass through such a
passageway 86B. In one embodiment, intervening magnetic structure fills
no more than 50% of a cross-sectional area between windings 88, 89, and
within passageway 86B.

[0099] In one embodiment, windings 88, 89 wind around teeth 86A of core
86, rather than around I-shaped core 87 or the non-teeth portion of core
86.

[0100]FIG. 10 shows a scalable, multi-phase coupled inductor 90 that may
be formed from a comb-shaped magnetic core 92 and an I-shaped (e.g., a
bar) magnetic core 93, in accord with one embodiment. Inductor 90 may,
for example, serve as inductor 28 of FIG. 1. The inductor 90 may be
formed by coupling "teeth" 92A of the comb-shaped core 92 to the I-shaped
core 93, similar to FIG. 9. The inductor 90 may be scaled to include more
phases by the addition of the one or more core cells to form a scalable
structure. In one embodiment, H-shaped cores 91 (such as those shown in
FIG. 6 as H-shaped magnetic cores 74) may be coupled to cores 92 and 93,
as shown. The windings 94 and 95 may be wound through the passageways 90A
formed by the teeth 92A as coupled with I-shaped core 93. The windings 94
and 95 may be wound about the teeth 92A of core 92 and the "bars" 91A of
H-shaped cores 91. Scalability and ease of manufacturing advantages are
similar to those previously mentioned. Those skilled in the art should
appreciate that other shapes, such as the U-shaped cores and rectangular
shaped cores, may be formed similarly to cores 92 and 93. Each of the
I-shaped core 92 and the H-shaped cores 91 has a respective planar
surface 92P and 91P, for example, that aligns in the same plane and
facilitates mounting of inductor 90 onto PCB 90S. At least part of one
passageway 90A is free from intervening magnetic structure between
windings 94, 95; air may for example fill the space of this passageway
90A and between windings 94, 95. By way of example, five passageways 90A
are shown each with a pair of windings 94, 95 passing therethrough, with
only air filling the space between the windings 94, 95. One winding 94 is
at the end of inductor 90 and does not pass through such a passageway
90A; and another winding 95 is at another end of inductor 90 and does not
pass through such a passageway 90A. In one embodiment, intervening
magnetic structure fills no more than 50% of a cross-sectional area
between windings 94, 95, and within passageway 90A.

[0101] FIGS. 11-13 show staple magnetic cores 102 that may serve to
implement a scalable multi-phase coupled inductor 100. Inductor 100 may,
for example, serve as inductor 28 of FIG. 1. The staple magnetic cores
102 are, for example, U-shaped and may function similar to a "staple".
The staple magnetic cores 102 may connect, or staple, through PCB 101 to
bus bars 103 to form a plurality of magnetic core cells. For example, the
two bus bars 103 may be affixed to one side of PCB 101 such that the
staple magnetic cores 102 traverse through the PCB 101 from the opposite
side of the PCB (e.g., via apertures 101H) to physically couple to the
bus bars 103. One staple magnetic core may implement a single phase for
the inductor 100; thus the inductor 100 may be scalable by adding more of
staple magnetic cores 102 and windings 104, 105. For example, a two-phase
coupled inductor would have two staple magnetic cores 102 coupled to bus
bars 103 with each core having a winding, such as windings 104, 105; the
number of phases are thus equal to the number of staple magnetic cores
102 and windings 104, 105. By way of example, inductor 100, FIG. 11,
shows a 3-phase inductor. Bus bars 103 may have center axes 402 and
staple magnetic cores 102 may have center axes 404.

[0102] Advantages of this embodiment provide a PCB structure that may be
designed in layout. As such, PCB real estate determinations may be made
with fewer restrictions, as the inductor 100 becomes part of the PCB
design. Other advantages of the embodiment are apparent in FIG. 13.
There, it can be seen that the staples 102 may connect to PCB 101 at
angles to each PCB trace (i.e., windings 104 and 105) so as to not incur
added resistance while at the same time improving adjustability of
leakage inductance. For example, extreme angles, such as 90 degrees, may
increase the overall length of a PCB trace, which in turn increases
resistance due to greater current travel distance. Further advantages of
this embodiment include the reduction or avoidance of solder joints,
which can significantly diminish high current. Additionally, the
embodiment may incur fewer or no additional winding costs as the windings
are part of the PCB; this may improve dimensional control so as to
provide consistent characteristics such as AC resistance and leakage
inductance.

[0103] Similar to coupled inductor 100, FIG. 14 shows bar magnetic cores
152, 153 that serve to implement a scalable coupled inductor 150.
Inductor 150 may, for example, serve as inductor 28 of FIG. 1. The bar
magnetic cores 152, 153 are, for example, respectively mounted to
opposing sides 156, 157 of PCB 151. Each of the bar magnetic cores 152,
153 has, for example, a respective planar surface 152P, 153P that
facilitates mounting of the bar magnetic cores to PCB 151. The bar
magnetic cores 152, 153, in this embodiment, do not physically connect to
each other but rather affix to the sides of 156, 157 such that coupling
of the inductor 150 is weaker. The coupling of the inductor 150 may,
thus, be determinant upon the thickness of the PCB 151; this thickness
forms a gap between cores 152 and 153. One example of a PCB that would be
useful in such an implementation is a thin polyimide PCB. One bar
magnetic core 152 or 153 may implement a single phase for the inductor
150; and inductor 150 may be scalable by adding additional bar magnetic
cores 152 or 153. For example, a two-phase coupled inductor has two bar
magnetic cores 152 coupled to two bus bars 153, each core having a
winding 154 or 155 respectively. The number of phases are therefore equal
to the number of bar magnetic cores 152, 153 and windings 154, 155. One
advantage of the embodiment of FIG. 14 is that no through-holes are
required in PCB 151. The gap between cores 152 and 153 slightly reduces
coupling so as to make the DC-to-DC converter system using coupled
inductor 150 more tolerant to DC current mismatch. Another advantage is
that all the cores 152, 153 are simple, inexpensive I-shaped magnetic
bars. Cores 152 may have center axes 408, and cores 153 may have center
axes 406.

[0104] FIGS. 15-16 each show a multi-phase coupled inductor (e.g., 110 and
120, respectively) with through-board integration, in accord with other
embodiments. FIG. 15 shows a coupled inductor 110 that may be formed from
a comb-shaped core 111 coupled to an I-shaped core 112 (e.g., a bar),
similar to that shown in FIG. 9. In this embodiment, the cores 111 and
112 may be coupled through PCB 113 and are integrated with PCB 113. The
windings 114, 115 may be formed in PCB 113 and/or as printed circuit
traces on PCB 113, or as wires connected thereto.

[0105] In FIG. 15, comb-shaped core 111 and I-shaped core 112 form a
series of passageways 117 within coupled inductor 110. At least part of
one passageway 117 is free from intervening structure between windings
114, 115; air may for example fill the space of this passageway 117 and
between windings 114, 115. By way of example, three passageways 117 are
shown each with a pair of windings 114, 115 passing therethrough, with
non-magnetic structure of PCB 113 filling some or all of the space
between the windings 114, 115. One winding 114 is at the end of inductor
110 and does not pass through such a passageway 117; and another winding
115 is at another end of inductor 110 and does not pass through such a
passageway 117. In one embodiment, intervening magnetic structure fills
no more than 50% of a cross-sectional area between windings 114, 115, and
within passageway 117.

[0106] FIG. 16 shows another through-board integration in a coupled
inductor 120. In this embodiment, magnetic cores 121 and 122 may be
coupled together by "sandwiching" the cores 121, 122 about PCB 123. The
connections to the cores 121, 122 may be implemented via holes 126 in the
PCB 123. The holes 126 may be filled with a ferromagnetic powder and/or
bar that couples the two cores together, when sandwiched with the PCB
123. Similarly, the windings 124, 125 may be formed in PCB 123 and/or as
printed circuit traces on PCB 123, or as wires connected thereto.
Inductors 110 and 120 may, for example, serve as inductor 28 of FIG. 1.
In the embodiment illustrated in FIG. 16, the windings 124 and 125 are
illustrated as PCB traces located within a center, or interior, plane of
the PCB 123. Those skilled in the art should readily appreciate that the
windings 124 and 125 may be embedded into any layer of the PCB and/or in
multiple layers of the PCB, such as exterior and/or interior layers of
the PCB.

[0107] In FIG. 16, cores 121 and 122 and ferromagnetic-filled holes 126
form a series of passageways 118 within coupled inductor 120. At least
part of one passageway 118 is free from intervening structure between
windings 124, 125; air may for example fill the space of this passageway
118 and between windings 124, 125. By way of example, three passageways
118 are shown each with a pair of windings 124, 125 passing therethrough,
with non-magnetic structure of PCB 123 filling some or all of the space
between the windings 124, 125. One winding 124 is at the end of inductor
120 and does not pass through such a passageway 118; and another winding
125 is at another end of inductor 120 and does not pass through such a
passageway 118. In one embodiment, intervening magnetic structure fills
no more than 50% of a cross-sectional area between windings 124, 125, and
within passageway 118.

[0108]FIG. 17 shows a multi-phase scalable coupled ring-core inductor
130, in accord with one embodiment. The inductor 130 may be formed from
multiple ring magnetic cores 131A, 131B, and 131C. In this embodiment,
cores 131A, 131B, and 131C may be coupled to one another. The ring
magnetic cores 131A, 131B, and 131C may have respective planar surfaces
131AP, 131BP, and 131CP, for example, that align in the same plane, to
facilitate mounting with electronics such as a PCB. Each core may have a
passageway 135 through which windings 132, 133, and 134 may be wound. As
one example, cores 131A and 131B may be coupled to one another as winding
133 may be wound through the passageways and about the cores. Similarly,
cores 131B and 131C may be coupled to one another as winding 132 may be
wound through the passageways 135 of those two cores. Cores 131C and 131A
may be coupled to one another as winding 134 is wound through the
passageways of those two cores. In another embodiment, the multiple ring
magnetic cores 131A, 131B, and 131C may be coupled together by windings
such that inductor 130 appears as a string or a chain. In one embodiment,
intervening magnetic structure fills no more than 50% of a
cross-sectional area between the windings within each respective
passageway 135.

[0109]FIG. 18 is a side perspective view and FIG. 19 is a top plan view
of one multi-phase coupled inductor 500. Inductor 500 may, for example,
serve as inductor 28 of FIG. 1. Inductor 500 is illustrated as being a
three phase coupled inductor; however, embodiments of inductor 500 may
support M phases, wherein M is an integer greater than one.

[0110] Inductor 500 includes core 502 and M windings 506, wherein each
winding may be electrically connected to a respective phase (e.g., a
phase 26 of FIG. 1) of a power converter (e.g., DC-to-DC converter system
10 of FIG. 1). Core 502 may be a single piece (e.g., a block core);
alternately, core 502 may be formed of two or more magnetic elements. For
example, core 502 may be formed of a comb-shaped magnetic element coupled
to an I-shaped magnetic element; as another example, core 502 may be
formed of a plurality of C-shaped magnetic elements or H-shaped magnetic
elements coupled together. Core 502 includes a bottom surface 508 (e.g.,
a bottom planar surface) and a top surface 510 opposite bottom surface
508. Core 502 has a first side 522 opposite a second side 524 and a third
side 548 opposite a fourth side 550 (labeled in FIG. 19).

[0111] Core 502 forms M-1 interior passageways 504. For example, inductor
500 is illustrated in FIGS. 18 and 19 as supporting three phases;
accordingly, core 502 forms two interior passageways 504(1) and 504(2).
Passageways 504 extend from top surface 510 to bottom surface 508. Core
502 further defines M legs 512. In FIGS. 18 and 19, legs 512(1), 512(2),
and 512(3) are partially delineated by dashed lines, which are included
for illustrative purposes and do not necessarily denote discontinuities
in core 502. Each passageway 504 is at least partially defined by two of
the M legs; for example, passageway 504(1) is partially defined by legs
512(1) and 512(2).

[0112] Core 500 has a width 526 (labeled in FIG. 19) and a height 528
(labeled in FIG. 18). Height 528 is, for example, 10 millimeters or less.
Passageways 504 also have height 528. Passageways 504 each have a width
530 and a depth 532 (labeled in FIG. 19). In an embodiment of inductor
500, a ratio of passageway width 530 to passageway depth 532 is at least
about 5.

[0113] As stated above, inductor 500 includes M windings 506, and inductor
500 is illustrated in FIGS. 18 and 19 as supporting three phases.
Accordingly, inductor 500 includes three windings 506(1), 506(2), and
506(3). M-2 of the M windings 506 are wound at least partially about a
respective leg of the magnetic core and through two of the M-1 interior
passageways. For example, in FIGS. 18 and 19, winding 506(2) is wound
partially about leg 512(2) and through passageways 504(1) and 504(2). Two
of the M windings are wound at least partially about a respective leg of
magnetic core 502 and through one interior passageway 504. For example,
in FIGS. 18 and 19, winding 506(1) is wound partially about leg 512(1)
and through passageway 504(1), and winding 506(3) is wound partially
about leg 512(3) and through passageway 504(2). Each passageway 504 has
two windings 506 wound therethrough, as may be observed from FIGS. 18 and
19.

[0114] Each passageway 504 may be at least partially free of intervening
magnetic structure between the two windings wound therethrough. For
example, as may be best observed from FIG. 19, in the embodiment of FIGS.
18 and 19, there is no intervening magnetic structure between windings
506(1) and 506(2) in passageway 504(1), and there is no intervening
magnetic structure between windings 506(2) and 506(3) in passageway
504(2).

[0115] Each of the two windings in a passageway 504 are separated by a
linear separation distance 534 (labeled in FIG. 19) in a plane parallel
to first side 522 and second side 524 of core 502. In an embodiment, a
ratio of separation distance 534 to passageway width 530 is at least
about 0.15.

[0116] Each winding 506 has two ends, wherein the winding may be
electrically connected to a circuit (e.g., a power converter) at each
end. Each end of a given winding extends from opposite sides of core 502.
For example, one end of winding 506(2) extends from side 522 of core 502
in the direction of arrow 538 (illustrated in FIG. 19), and the other end
of winding 506(2) extends from side 524 of core 502 in the direction of
arrow 540 (illustrated in FIG. 19). Such configuration of inductor 500
may allow each winding 506 to connect to a respective switching node
proximate to one side (e.g., side 522 or 524) of inductor 500 and each
winding 506 to connect to a common output node on an opposite side (e.g.,
side 524 or 522) of inductor 500. Stated differently, the configuration
of inductor 500 may allow all switching nodes to be disposed adjacent to
one side of inductor 500 and the common output node to be disposed on the
opposite side of inductor 500. For example, each winding end extending
from side 522 of core 502 may connect to a respective switching node, and
each winding end extending from side 524 of core 502 may connect to a
common output node. Lengths of windings 506 and/or external conductors
(e.g., printed circuit board traces or bus bars) may advantageously be
reduced by disposing all switching nodes on one side of inductor 500 and
the common output node on the opposite side of inductor 500. Reducing the
length of windings 506 and/or external conductors may reduce the
resistance, cost, and/or size of inductor 500 and/or an external circuit
(e.g., a power converter) that inductor 500 is installed in.

[0117] In an embodiment, windings 506 have rectangular cross section as
illustrated in FIGS. 18 and 19. In such embodiment, each winding 506
forms at least three planar sections 542, 544, and 546. For example,
winding 506(1) forms planar sections 542(1), 544(1), and 546(1). Planar
sections 542 and 546 are about parallel with each other, and planar
sections 542 and 546 are about orthogonal to planar section 544. Planar
sections 542 and 546 may also be about parallel to bottom surface 508.

[0118] In an embodiment, each winding 506 has a first end forming a first
tab 514 and a second end forming a second tab 518, as illustrated in
FIGS. 18 and 19. First and second tabs 514, 518 are, for example,
integral with their respective windings, as illustrated in FIGS. 18 and
19. For example, winding 506(1) of FIG. 18 forms first tab 514(1) and
second tab 518(1). Each first tab 514 for example forms a first surface
516 (e.g., a planar surface) parallel to bottom surface 508, and each
second tab 518 for example forms a second surface 520 (e.g., a planar
surface) about parallel to bottom surface 508. For example, first tab
514(3) forms first surface 516(3) and second tab 518(3) forms second
surface 520(3). Each first surface 516 and second surface 520 may be used
to connect its respective tab to a printed circuit board disposed
proximate to bottom surface 508. M-1 of first tabs 514 and M-1 of second
tabs 518 are each at least partially disposed along bottom surface 508;
for example, in FIGS. 18 and 19, first tabs 514(2) and 514(3) are
partially disposed along bottom surface 508, and second tabs 518(1) and
518(2) are partially disposed along bottom surface 508.

[0119] Core 502 and each winding 506 collective form a magnetizing
inductance of inductor 500 as well as a leakage inductance of each
winding 506. As discussed above with respect to FIG. 1, the leakage
inductance of each winding, for example, ranges from 10 nH to 200 nH.
Furthermore, separation distance 534 between adjacent windings may be
chosen to be sufficiently large such that the leakage inductance of each
winding 506 is sufficiently large. Separation distance 534 is, for
example, 1.5 millimeters or greater (e.g., 3 millimeters). In embodiments
of inductor 500, the magnetizing inductance of inductor 500 is greater
than the leakage inductance of each winding 506.

[0120]FIG. 20 is a top plan view of a two-phase coupled inductor 500(1),
which is a two-phase embodiment of inductor 500 of FIGS. 18 and 19. As
illustrated in FIG. 20, core 502(1) includes legs 512(4) and 512(5). Leg
512(4) extends from first side 522(1) to second side 524(1) and defines
third side 548(1); leg 512(5) extends from first side 522(1) to second
side 524(1) and defines fourth side 550(1). Interior passageway 504(3)
extends from a top surface 510(1) to a bottom surface of core 502(1) (not
visible in the top plan view of FIG. 20). Winding 506(4) is wound
partially about leg 512(4), through interior passageway 504(3), and along
third side 548(1). Winding 506(5) is wound partially about leg 512(5),
through interior passageway 504(3), and along fourth side 550(1).

[0121] Windings 506(4) and 506(5) each form a first end for connecting the
winding to a respective switching node of a power converter. The first
end of winding 506(4) forms a first tab 514(4), and the first end of
winding 506(5) forms a first tab 514(5). Each of first tabs 514(4) and
514(5) for example has a surface about parallel to the bottom surface of
core 502(1) for connecting the first tab to a printed circuit board
disposed proximate to the bottom surface of core 502(1). Each of first
tabs 514(4) and 514(5) extends beyond core 502(1) from first side 522(1)
of the core in the direction indicated by arrow 552.

[0122] Windings 506(4) and 506(5) each form a second end for connecting
the winding to a common output node of the power converter. The second
end of winding 506(4) forms a second tab 518(4), and the second end of
winding 506(5) forms a second tab 518(5). Each of second tabs 518(4) and
518(5) has for example a surface about parallel to the bottom surface of
core 502(1) for connecting the second tab to the printed circuit board
disposed proximate to the bottom surface of core 502(1). Each of second
tabs 518(4) and 518(5) extends beyond core 502(1) from second side 524(1)
of the core in the direction indicated by arrow 554.

[0123]FIG. 21 is a side perspective view of one multi-phase coupled
inductor 600. Inductor 600 is essentially the same as an embodiment of
inductor 500 having windings 506 with rectangular cross section with the
exception that windings 506 of inductor 600 form at least five planar
sections 604, 606, 608, 610, and 612. It should be noted that each of the
five planar sections are not visible for each winding 506 in the
perspective view of FIG. 21. For example, winding 506(8) of inductor 600
forms planar sections 604(3), 608(3), 610(1), and 612(3) as well as an
additional planar section that is not visible in the perspective view of
FIG. 21. Such additional planar section of winding 506(8) corresponds to
planar section 606(1) of winding 506(6). Planar sections 604, 608, and
612 are, for example, about parallel to a bottom surface 508(2) of core
502(2). Forming windings 506 with at least five planar sections may
advantageously reduce a height 602 of inductor 600.

[0124] Power is lost in a coupled inductor's windings as current flows
through the windings. Such power loss is often undesirable for reasons
including (a) the power loss can cause undesired heating of the inductor
and/or the system that the inductor is installed in, and (b) the power
loss reduces the system's efficiency. Power loss in a coupled inductor
may be particularly undesirable in a portable system (e.g., a notebook
computer) due to limited capacity of the system's power source (e.g.,
limited capacity of a battery) and/or limitations in space available for
cooling equipment (e.g., fans, heat sinks) Accordingly, it would be
desirable to reduce power loss in a coupled inductor's windings.

[0125] One reason that power is lost as current flows through a coupled
inductor's winding is that such winding is formed of a material (e.g.,
copper or aluminum) that is not a perfect electrical conductor. Stated
differently, such material that the winding is formed of has a non-zero
resistivity, and accordingly, the winding has a non-zero resistance. This
resistance is commonly referred to as DC resistance, or ("RDC"), and
is a function of characteristics including the winding's length, cross
sectional area, temperature, and resistivity. Specifically, RDC is
directly proportional to the winding's length and its constituent
material's resistivity; conversely, RDC is indirectly proportional
to the winding's cross sectional area. Power loss due to DC resistance
("PDC") is given by the following equation:

PDC=RDCI2 ,EQN. 1

where I is either the magnitude of direct current flowing through the
winding, or the root mean square ("RMS") magnitude of AC current flowing
through the winding. Accordingly, PDC may be reduced by reducing
RDC.

[0126] Another reason that power may be lost as current flows through a
coupled inductor's winding is that the winding has a non-zero AC
resistance ("RAC"). RAC is an effective resistance resulting
from AC current flowing through the winding, and RAC increases with
increasing frequency of AC current flowing through the winding. Power
loss due to RAC is zero if solely direct current flows through the
winding. Accordingly, if solely direct current flows through a winding,
power is lost in the winding due to the winding having a non-zero
RDC, but no additional power is lost in the winding due to RAC.
However, under AC conditions, power is lost in a winding due to both
RAC and RDC having non-zero values. For the purposes of this
disclosure and corresponding claims, alternating current includes not
only sinusoidal current having a single frequency, but also any current
that varies as a function of time (e.g., a current waveform having a
fundamental frequency and a plurality of harmonics such as a triangular
shaped current waveform). Accordingly, it would be desirable to minimize
both RAC and RDC of a coupled inductor intended to conduct AC
current in order to minimize power lost in the inductor's windings.

[0127] Inductors installed in DC-to-DC converters, such as DC-to-DC
converter system 10 of FIG. 1, commonly conduct alternating currents. The
frequency of such alternating currents is often relatively high, such as
in the tens to hundreds of kilohertz, or even in the megahertz range.
Accordingly, RAC may result in significant power loss in inductors
(e.g., coupled inductor 28) used in DC-to-DC converters.

[0128] One contributor to RAC is commonly called the skin effect. The
skin effect describes how alternating current tends to be
disproportionately distributed near the surface of a conductor (e.g., the
outer surface of a winding). The skin effect becomes more pronounced as
the current's frequency increases. Accordingly, as the frequency of
current flowing through a conductor increases, the skin effect causes a
reduced portion of the conductor's cross sectional area to be available
to conduct current, and the conductor's effective resistance thereby
increases.

[0129] A conductor's inductance may also contribute to its RAC.
Current flowing through a conductor (e.g., a winding) will tend to travel
along the path that results in the least inductance. If a conductor is
not completely linear (e.g., a winding wound around a magnetic core),
current will tend to flow through the conductor in a manner that creates
the smallest loop and thereby minimizes inductance. Thus, as the
frequency of current flowing through the conductor increases, inductance
causes a reduced portion of the conductor's cross sectional area to be
available to conduct current, and the conductor's effective resistance
thereby increases.

[0130] The effects of RAC may be appreciated by referring to FIGS. 22
and 23. FIG. 22 is a top plan view of one inductor winding 2200. Winding
2200 has inner sides 2202 and opposite outer sides 2204. Under AC
operating conditions, current flowing through winding 2200 will not be
evenly distributed along width 2206 of winding 2200. Instead, current
flowing through winding 2200 will be most densely distributed closest to
inner sides 2202 and least densely distributed closest to outer sides
2204. Such non-uniform distribution of current flowing through winding
2200, which is due to both the skin effect and inductance of winding
2200, increases the conductor's effective resistance by reducing the
cross-sectional area of winding 2200 being utilized to carry current.
Accordingly, winding 2200 has a non-zero value of RAC, which causes
power loss in winding 2200 to increase in proportion to the frequency of
current flowing through winding 2200.

[0131]FIG. 23 is a top perspective view of one foil winding 2200(1),
which is an embodiment of winding 2200 of FIG. 22. Winding 2200(1) has
width 2206(1) and thickness 2302. As can be observed from FIG. 23, width
2206(1) has a value that is significantly greater than the value of
thickness 2302. Accordingly, top surface area 2304 of winding 2200(1) is
significantly greater than combined surface area of inner sides 2202(4),
2202(5), and 2202(6).

[0132] In the same manner as that discussed above with respect to FIG. 22,
alternating current flowing through winding 2200(1) will be most heavily
distributed closest to inner sides 2202 and least heavily distributed
closest to outer sides 2204. Because width 2206(1) is significantly
greater than thickness of 2302, a significant portion of the cross
section 2306 of winding 2200(1) may be underutilized when winding 2200(1)
is carrying alternating current. Accordingly, winding 2200(1) is likely
to have an RAC value larger than that expected from the skin effect
alone.

[0133]FIG. 24 is a top perspective view of one M-phase coupled inductor
2400, where M is an integer greater than one. Coupled inductor 2400 may,
for example, serve as inductor 28 of FIG. 1. Coupled inductor 2400 is
designed such that its windings advantageously have a low RDC and
RAC, as discussed below. Although coupled inductor 2400 is
illustrated in FIG. 24 as having two phases, embodiments of inductor 2400
have greater than two phases. For example, coupled inductor 2400(1)
illustrated in FIG. 25, which is discussed below, has three phases.

[0134] Coupled inductor 2400 includes a magnetic core having end magnetic
elements 2408 and 2410 as well as M legs 2404. Legs 2404 are disposed
between end magnetic elements 2408 and 2410, and legs 2404 connect end
magnetic element 2408 and 2410. Each leg 2404 has a width 2402 equal to a
linear separation distance between end magnetic elements 2408 and 2410
where the end magnetic elements are connected by the leg. Stated
differently, each leg 2404 has a respective width 2402 in the direction
connecting end magnetic elements 2408 and 2410. Each leg 2404 may have
the same width 2402; alternately, width 2402 may vary among legs 2404 in
coupled inductor 2400.

[0135] Each leg 2404 has an outer surface 2406. Outer surface 2406 may
include a plurality of sections. For example, FIG. 24 illustrates legs
2404 having a rectangular shape such that the outer surface of each leg
2404 includes four planar sections, one of such four planar sections
being a bottom planar surface. In the perspective view of FIG. 24, only
two of the planar sections of outer surface 2406 of each leg 2404 are
visible. For example, the bottom planar surface of each leg 2404 is not
visible in the perspective view of FIG. 24.

[0136] Coupled inductor 2400 may have legs 2404 formed in shapes other
than rectangles. For example, in an embodiment of coupled inductor 2400
(not shown in FIG. 24), legs 2404 have an outer surface 2406 including a
planar first surface and a rounded second surface.

[0137] The core of coupled inductor 2400 is formed, for example, of a
ferrite material including a gap filled with a non-magnetic material
(e.g., air) to prevent coupled inductor 2400 from saturating. As another
example, the core of coupled inductor 2400 may be formed of a powdered
iron material, a Kool-μ® material, or similar materials commonly
used for the manufacturing of magnetic cores for magnetic components.
Powered iron may be used, for example, if coupled inductor 2400 is to be
used in relatively low frequency applications (e.g., 250 KHz or less).
Although FIG. 24 illustrates end magnetic elements 2408 and 2410 as well
as legs 2404 as being discrete elements, one or more of such elements may
be combined. Furthermore, at least one of end magnetic elements 2408 and
2410 as well as legs 2404 may be divided. For example, the core of
coupled inductor 2400 may be formed from a comb-shaped and an I-shaped
magnetic element.

[0138] As noted above, coupled inductor 2400 is illustrated in FIG. 24 as
having two phases; accordingly, coupled inductor 2400 has two legs 2404
in FIG. 24. FIG. 25 is a top perspective view of one coupled inductor
2400(1), which is a three phase embodiment of coupled inductor 2400.
Coupled inductor 2400(1) includes three legs 2404(1), 2404(2), and
2404(3) connecting end magnetic elements 2408(1) and 2410(1).

[0139] Coupled inductor 2400 includes M windings, each of which are
magnetically coupled to each other. Each winding is wound at least
partially about a respective leg 2404. Each winding may form a single
turn or a plurality of turns, and may include solder tabs for connecting
the winding to a PCB. Windings are not shown in FIGS. 24 and 25 in order
to promote illustrative clarity. In some embodiments of coupled inductor
2400, at least one section of outer surface 2406 is substantially covered
by a winding.

[0140] FIG. 26 is a side perspective view of one winding 2600, which is an
embodiment of a winding that may be used with coupled inductor 2400. As
discussed above, coupled inductor 2400 includes M windings; accordingly,
an embodiment of coupled inductor 2400 including windings 2600 will
include M windings 2600, where each winding 2600 is at least partially
wound about a respective leg 2404. Windings 2600, for example, form a
single turn, as illustrated in FIG. 26. However, other embodiments of
windings 2600 may form multiple turns; such multi-turn windings may be
electrically insulated using a dielectric tape, a dielectric coating, or
other insulating material to prevent turns from electrically shorting
together.

[0141] Winding 2600 for example has a substantially rectangular cross
section. In the context of this disclosure and corresponding claims,
windings having a substantially rectangular cross section include, but
are not limited to, foil windings. Each winding 2600 has an inner surface
2602, an opposite outer surface 2606, width 2608, and thickness 2604 that
is orthogonal to inner surface 2602 and outer surface 2606. Width 2608
is, for example, greater than (e.g., at least two or five times)
thickness 2604. Thus, some embodiments of winding 2600 have an aspect
ratio (ratio of width 2608 to thickness 2604) of at least two or five. As
discussed below, such characteristics help reduce each winding 2600's
RAC. When winding 2600 is wound about a respective leg 2404, width
2608 is parallel to width 2402 of the respective leg. Embodiments of
winding 2600 have a value of width 2608 that is, for example, at least
eighty percent of the value of width 2402 of the respective leg 2404 that
the winding is wound about. For example, winding 2600 may have a width
2608 that is about equal to the value of width 2402 of the leg that the
winding is wound at least partially about.

[0142] Winding 2600 has a first end 2614 and a second end 2616; first end
2614 and second end 2616 may form respective solder tabs for connecting
winding 2600 to a PCB. For example, winding 2600 is illustrated in FIG.
26 as including solder tabs 2610 and 2612, each having a common width
2620 that is equal to width 2608 of winding 2600. Solder tabs 2610 and
2612 are, for example, integral with winding 2600 as illustrated in FIG.
26. If an embodiment of winding 2600 having solder tabs is wound about a
leg 2404 having a bottom planar surface, the solder tabs may be disposed
along such bottom planar surface.

[0143] Winding 2600 has a cross section 2618 orthogonal to winding 2600's
length. Cross section 2618 is, for example, rectangular. Winding 2600 is
illustrated in FIG. 26 as being formed into five rectangular sections.
Accordingly, each of inner surface 2602 and outer surface 2606 includes
five different rectangular sections, although not all of such sections
are visible in the perspective view of FIG. 26. However, winding 2600 may
have fewer than five sections (e.g., if it does not include solder tabs),
or greater than five sections (e.g., if it is a multi-turn winding).

[0144] When coupled inductor 2400 includes M windings 2600, each of the M
windings 2600 is wound about a respective leg 2404 such that inner
surface 2602 of the winding is wound about the outer surface 2406 of the
leg. Stated differently, inner surface 2602 of winding 2600 faces outer
surface 2406 of the leg. For example, FIG. 27 is a side plan view of one
leg 2404(4) having a winding 2600(1) partially wound about. As can be
observed from FIG. 27, winding 2600(1) is a single turn winding and inner
surface 2602(1) of winding 2600(1) is wound about outer surface 2406(1)
of leg 2404(4).

[0145]FIG. 28 is a bottom perspective view of winding 2600(2), which is
an embodiment of winding 2600 before it has been wound about a leg 2404.
Winding 2600(2) has width 2608(1) and thickness 2604(1), where thickness
2604(1) is orthogonal to inner surface 2602(2). Width 2608(1) is greater
than (e.g., at least two or five times) thickness 2604(1). Embodiments of
winding 2600(2) have width 2608(1) being at least two millimeters. Cross
section 2618(2), which is orthogonal to a length 2802, is visible in FIG.
28. As can be observed from FIG. 28, the surface area of inner surface
2602(2) is greater than the surface area of cross section 2618(2).

[0146]FIG. 29 is a top perspective view of one coupled inductor 2400(2),
which is another embodiment of coupled inductor 2400 of FIG. 24. Coupled
inductor 2400(2) includes single turn windings 2600(3) and 2600(4)
partially wound about respective legs 2404(5) and 2404(6). Legs 2404(5)
and 2404(6) each have a rectangular shape having an outer surface
including four planar sections, and three of the four planar sections of
each leg are substantially covered by the leg's respective winding.
Furthermore, legs 2404(5) and 2404(6) as well as windings 2600(3) and
2600(4) each have a common width 2904. Width 2904 is, for example, at
least 1.5 millimeters. End magnetic element 2410(2) is illustrated as
being partially transparent in FIG. 29 in order to show ends 2902(1) and
2902(2) of windings 2600(3) and 2600(4), respectively. Although coupled
inductor 2400(2) is illustrated in FIG. 29 as having two phases, coupled
inductor 2400(2) may have greater than two phases.

[0147]FIG. 30 is a top plan view of one coupled inductor 2400(3), which
is another embodiment of coupled inductor 2400 of FIG. 24. Coupled
inductor 2400(3) includes end magnetic elements 2408(3) and 2410(3) as
well as legs 2404(7) and 2404(8). Coupled inductor 2400(3) is shown in
FIG. 30 with dimensions specified in millimeters. However, it should be
noted that the dimensions of coupled inductor 2400(3) are exemplary and
may be varied as a matter of design choice. Coupled inductor 2400(3) may
have, for example, a relatively small width 3006 of about 13 millimeters.

[0148]FIG. 31 is a plan view of side 3002 of coupled inductor 2400(3) of
FIG. 30. Elements visible in FIG. 31 include outlines of single turn
windings 2600(5) and 2600(6), which are represented by dashed lines.
Windings 2600(5) and 2600(6) are not shown in FIG. 30 in order to promote
clarity. FIG. 32 is a plan view of side 3004 of coupled inductor 2400(3).

[0149] FIG. 33 is a top plan view of one PCB layout 3300. PCB layout 3300,
which advantageously offers relatively low conduction losses as discussed
below, may be used with embodiments of coupled inductor 2400 of FIG. 24
including windings 2600. Although the embodiment of layout 3300
illustrated in FIG. 33 is for a two phase embodiment of coupled inductor
2400, layout 3300 may be extended to three or more phases.

[0150] Layout 3300 includes one pad 3302 for a first terminal (e.g.,
solder tab 2610, FIG. 26) of each winding 2600. The configuration of
coupled inductor 2400 including windings 2600 allows pads 3302 to be
relatively small and thereby connect to relatively large respective
switching node shapes 3306. The relatively large surface area of each
switching node shape 3306 causes it to have a relatively low resistance,
which helps minimize conduction losses resulting from current flowing
therethrough.

[0151] Layout 3300 further includes one pad 3304 for a second terminal
(e.g., solder tab 2612, FIG. 26) of each winding 2600. As with pads 3302,
the configuration of coupled inductor 2400 with windings 2600 allows pads
3304 to be relatively small and thereby connect to a relatively large
common output node shape 3308. The relatively large surface area of
common output node shape 3308 causes it to have a relatively low
resistance, which thereby helps minimize conduction losses when current
flows therethrough. Furthermore, the relatively small size of pads 3304
allows a large number of vias 3310 (only some of which are labeled for
illustrative clarity) connecting output node shape 3308 to one or more
internal PCB layers to advantageously be disposed relatively close to
pads 3304. Disposing a large number of vias 3310 close to pads 3304
further helps minimize conduction losses by providing a low resistance
path between the coupled inductor and the one or more internal PCB
layers.

[0152] In contrast to coupled inductor 2400 including windings 2600, some
other coupled inductors require relatively large pads for connecting the
inductor to a PCB. In many coupled inductor applications, the amount of
PCB surface area available for mounting a coupled inductor is limited.
The relatively large surface area required by the pads for the other
coupled inductors reduces the amount of PCB surface area available for
the shapes (e.g., shapes performing functions similar to those of 3306
and 3308) connected to such pads. Accordingly, such shapes of layouts for
the other coupled inductors may have a higher resistance (and therefore a
higher conduction loss) than shapes 3306 and 3308 of layout 3300.

[0153] Layout 3300 has dimensions appropriate for the embodiment of
coupled inductor 2400 to be installed thereon. For example, in one
embodiment of layout 3300, dimension 3312 is about 13 millimeters ("mm"),
and dimension 3318 is about 2.5 mm. As another example, in another
embodiment of layout 3300, dimension 3312 is about 17 mm, dimension 3322
is about 3 mm, dimension 3318 is about 2.5 mm, dimension 3320 is about 1
mm, and dimension 3324 is about 19 mm. However, it should be noted that
such exemplary dimensions may be varied as a matter of design choice.

[0154] Some embodiments of coupled inductor 2400 have a relatively small
width (e.g., width 3006, FIG. 30) which allows embodiments of layout 3300
to have a relatively small width 3312, such as 13 millimeters. Such small
width advantageously reduces the distances current must flow across the
coupled inductor and its layout as represented by arrows 3314 and 3316.
Minimizing the distance current must flow in the PCB and the coupled
inductor helps reduce conduction losses, especially losses in conductors
of the PCB.

[0155]FIG. 34 is a side perspective view of another winding 3400, which
may be used in embodiments of coupled inductor 2400. Winding 3400, for
example, has a substantially rectangular cross section. Winding 3400
includes an inner surface 3402 and an opposite outer surface 3406. It
should be noted that only part of inner surface 3402 and outer surface
3406 are visible in the perspective view of FIG. 34. When windings 3400
are used in embodiments of coupled inductor 2400, inner surface 3402 of
each winding 3400 is wound about an outer surface 2406 of a respective
leg 2404. Thus, inner surface 3402 of each winding 3400 faces outer
surface 2406 of the respective leg that the winding 3400 is wound at
least partially about.

[0156] Winding 3400 has a width 3408 and a thickness 3404 orthogonal to
inner surface 3402. Width 3408 is, for example, greater (e.g., at least
two or five times greater) than thickness 3404. Thus, in some embodiments
of winding 3400, the aspect ratio of winding 3400's cross section is at
least two or at least five. When winding 3400 is wound about a respective
leg 2404, winding 3400's width 3408 is for example parallel to and at
least eighty percent of width 2402 of the leg. For example, winding
3400's width 3408 may be about equal to width 2402 of its respective leg
2404. Although winding 3400 is illustrated as forming a single turn,
winding 3400 may form a plurality of turns and thereby be a multi-turn
winding.

[0157] Winding 3400 may include two solder tabs 3410 and 3412, each having
respective widths 3420(1) and 3420(2) parallel to width 3408 of winding
3400. Each of widths 3420(1) and 3420(2) are less than one half of width
3408 in order to prevent solder tabs 3410 and 3412 from touching and
thereby electrically shorting. Solder tabs 3410 and 3412 may extend along
the majority of depth 3414 of winding 3400, such feature may
advantageously increase the surface area of a connection between solder
tabs 3410 and 3412 and a PCB that winding 3400 is connected to. Solder
tabs 3410 and 3412 are, for example, integral with winding 3400 as
illustrated in FIG. 34.

[0158] Winding 3400 may be wound about a leg 2404 having a rectangular
shape. In such case, winding 3400 will have five rectangular sections
(including solder tabs 3410 and 3412) as illustrated in FIG. 34. However,
winding 3400 could have a non-rectangular shape (e.g., a half circle) if
wound about an embodiment of leg 2404 having a non-rectangular shape.

[0159]FIG. 35 is a top plan view of winding 3400(1), which is an
embodiment of winding 3400 before being wound at least partially about a
leg 2404 of coupled inductor 2400. The dashed lines in FIG. 35 indicate
where winding 3400(1) would be folded if it were wound about a
rectangular embodiment of leg 2404; in such case, winding 3400 would have
rectangular sections 3502, 3504, and 3506 in addition to solder tabs
3410(1) and 3412(1) after being wound about the leg.

[0160]FIG. 36 is a side perspective view showing how an embodiment of
coupled inductor 2400 using windings 3400 could interface with a printed
circuit board. Specifically, FIG. 36 shows coupled inductor 2400(4)
disposed above solder pads 3602 and 3604. Although coupled inductor
2400(4) is illustrated as having two phases, coupled inductor 2400(4)
could have greater than two phases.

[0161] Coupled inductor 2400(4) includes one instance of winding 3400 for
each phase; however, windings 3400 are not shown in FIG. 36 in order to
promote illustrative clarity. Arrows 3606 indicate how solder tabs 3410
and 3412 (not shown in FIG. 36) would align with solder pads 3602 and
3604, respectively. Solder pads 3602(1) and 3602(2) connect to a common
output node, and solder pads 3604(1) and 3604(2) connect to respective
switching nodes.

[0162] FIG. 37 is a top plan view of one PCB layout 3700, which may be
used with embodiments of coupled inductor 2400 including windings 3400
(e.g., coupled inductor 2400(4) of FIG. 36). Although layout 3700 is
illustrated as supporting two phases, other embodiments of layout 3700
may support greater than two phases.

[0163] Layout 3700 includes pads 3702(1) and 3702(2) for connecting solder
tabs 3412 of windings 3400 to respective inductor switching nodes. Each
of pads 3702(1) and 3702(2) is connected to a respective switching node
shape 3704(1) and 3704(2). Layout 3700 further includes pads 3706(1) and
3706(2) for connecting solder tabs 3410 of windings 3400 to a common
output node. Each of pads 3706(1) and 3706(2) is connected to a common
output node shape 3708; shape 3708 may be connected to another layer of
the PCB using vias 3710 (only some of which are labeled for clarity).
Dimensions 3716 and 3718 are, for example, 5 millimeters and 17
millimeters respectively.

[0165]FIG. 38 is a side perspective view of one winding 3800, which may
be used in embodiments of coupled inductor 2400. Winding 3800 has, for
example, a substantially rectangular cross section. Winding 3800 includes
an inner surface 3802 and an opposite outer surface 3806. It should be
noted that only part of inner surface 3802 and outer surface 3806 are
visible in the perspective view of FIG. 38. When windings 3800 are used
in embodiments of coupled inductor 2400, the inner surface 3802 of each
winding 3800 is wound about an outer surface 2406 of a respective leg
2404. Thus, inner surface 3802 of winding 3800 faces outer surface 2406
of the respective leg that the winding is wound at least partially about.

[0166] Winding 3800 has a width 3808 and a thickness 3804 orthogonal to
inner surface 3802. Width 3808 is, for example, greater (e.g., at least
two or five times greater) than thickness 3804. Accordingly, some
embodiments of winding 3800 have an aspect ratio of at least two or at
least five. When winding 3800 is wound about a respective leg 2404,
winding 3800's width 3808 is for example parallel to and is least eighty
percent of width 2402 of the leg. For example, width 3808 may be about
equal to width 2402 of its respective leg. Although winding 3800 is
illustrated as forming single turn, winding 3800 may form a plurality of
turns and thereby be a multi-turn winding.

[0167] Winding 3800 may include two solder tabs 3810 and 3812. Solder tab
3810 extends away from winding 3800 in the direction indicated by arrow
3814, and solder tab 3812 extends away from winding 3800 in the direction
indicated by arrow 3816. Thus, solder tabs 3810 and 3812 extend beyond
winding 3800 in a direction parallel to width 3808 of winding 3800.
Solder tabs 3810 and 3812 may extend along the majority of depth 3818 of
winding 3800, such feature may advantageously increase the surface area
of a connection between solder tabs 3810 and 3812 and a PCB that winding
3800 is connected to. Solder tabs 3810 and 3812 are, for example,
integral with winding 3800 as illustrated in FIG. 38.

[0168] Winding 3800 may be wound about a leg 2404 having a rectangular
shape. In such case, winding 3800 will have five rectangular sections
(including solder tabs 3810 and 3812) as illustrated in FIG. 38. However,
winding 3800 could have a non-rectangular shape (e.g., a half circle) if
wound about an embodiment of leg 2404 having a non-rectangular shape.

[0169]FIG. 39 is a top plan view of winding 3800(1), which is an
embodiment of winding 3800 before being wound at least partially about a
leg 2404 of coupled inductor 2400. The dashed lines in FIG. 39 indicate
where winding 3800(1) would be folded if it were wound about a
rectangular embodiment of leg 2404; in such case, winding 3800 would have
rectangular sections 3902, 3904, and 3906 in addition to solder tabs
3810(1) and 3812(1) after being wound about the leg.

[0170]FIG. 40 is a side perspective view showing how an embodiment of
coupled inductor 2400 including windings 3800 could interface with a
printed circuit board. In particular, FIG. 40 shows coupled inductor
2400(5) disposed above solder pads 4002 and 4004. Although coupled
inductor 2400(5) is illustrated as having two phases, coupled inductor
could have greater than two phases.

[0171] Coupled inductor 2400(5) includes one instance of winding 3800 for
each phase. However, the windings are not shown in FIG. 40 in order to
promote clarity. Arrows 4006 indicate how solder tabs 3810 and 3812 (not
shown in FIG. 40) would align with solder pads 4002 and 4004,
respectively. Solder pads 4002(1) and 4002(2) connect to a common output
node, and solder pads 4004(1) and 4004(2) connect to respective switching
nodes.

[0172] FIG. 41 is a top plan view of one printed circuit board layout
4100, which may be used with embodiments of coupled inductor 2400
including windings 3800 (e.g., coupled inductor 2400(5) of FIG. 40).
Although layout 4100 is illustrated as supporting two phases, other
embodiments of layout 4100 may support more than two phases.

[0173] Layout 4100 includes pads 4102(1) and 4102(2) for connecting solder
tabs 3812 of windings 3800 to respective switching nodes. Each of pads
4102(1) and 4102(2) is connected to a respective switching node shape
4104(1) and 4104(2). Layout 4100 further includes pads 4106(1) and
4106(2) for connecting solder tabs 3810 of windings 3800 to a common
output node. Each of pads 4106(1) and 4106(2) is connected to a common
output node shape 4108; shape 4108 may be connected to another layer of
the PCB using vias 4110 (only some of which are labeled for clarity).
Dimensions 4116 and 4118 are, for example, 5 millimeters and 17
millimeters respectively.

[0175]FIG. 42 is a side perspective view of one winding 4200, which may
be used in embodiments of coupled inductor 2400. Winding 4200 is a
multi-turn winding. Although winding 4200 is illustrated in FIG. 42 as
forming two turns, winding 4200 can form more than two turns.

[0176] Winding 4200, for example, has a substantially rectangular cross
section. Winding 4200 includes an inner surface 4202 and an opposite
outer surface 4206. It should be noted that only part of inner surface
4202 and outer surface 4206 are visible in the perspective view of FIG.
42. When windings 4200 are used in embodiments of coupled inductor 2400,
the inner surface 4202 of each winding 4200 is wound about an outer
surface 2406 of a respective leg 2404. Thus, inner surface 4202 of
winding 4200 faces outer surface 2406 of the respective leg that the
winding is wound at least partially about.

[0177] Winding 4200 has a width 4208 and a thickness 4204 orthogonal to
inner surface 4202. Width 4208 is greater (e.g., at least two or five
times greater) than thickness 4204. Accordingly, some embodiments of
winding 4200 have an aspect ratio of at least two or at least five.
Winding 4200 is, for example, formed of a metallic foil.

[0178] Winding 4200 may further include solder tabs 4210 and 4212 for
connecting winding 4200 to a printed circuit board. Solder tabs 4210 and
4212 are, for example, rectangular and extend along a bottom surface of a
respective leg 2404 that the winding 4200 is wound at least partially
about. Additionally, solder tabs 4210 and/or 4212 may be extended (not
shown in FIG. 42) to increase printed circuit board contact area. Solder
tabs 4210 and 4212 are, for example, integral with winding 4200.

[0179]FIG. 43 is a side perspective view showing how an embodiment of
coupled inductor 2400 including windings 4200 could interface with a
printed circuit board. In particular, FIG. 43 shows coupled inductor
2400(6) disposed above solder pads 4302 and 4304. Coupled inductor
2400(6) is illustrated in FIG. 43 with end magnetic element 2410(4) being
transparent in order to show windings 4200(1) and 4200(2). Although
coupled inductor 2400(6) is illustrated as having two phases, coupled
inductor 2400(6) could have greater than two phases. In coupled inductor
2400(6), winding 4200(1) extends diagonally across a portion of outer
surface 4308(1) of leg 2404(9), and winding 4200(2) extends diagonally
across a portion of outer surface 4308(2) of leg 2404(10).

[0180] Arrows 4306 indicate how solder tabs 4210(1) and 4210(2) would
align with respective solder pads 4302(1) and 4302(2) and how solder tabs
4212(1) and 4212(2) would align with respective solder pads 4304(1) and
4304(2). Solder pads 4302(1) and 4302(2) connect to a common output node,
and solder pads 4304(1) and 4304(2) connect to respective switching
nodes.

[0181] As discussed above, each winding (e.g., winding 2600, 3400, 3800,
or 4200) of coupled inductor 2400 is at least partially wound about a
respective leg 2404 such that each winding's inner surface is adjacent to
outer surface 2406 of the respective leg. Accordingly, the inner surface
of the winding forms the smallest loop within the winding. However, as
noted above, each winding's width may be greater than the winding's
thickness. For example, winding 2600's width 2608 is greater than its
thickness 2604. Therefore, each winding is configured such that a
significant portion of its cross-sectional area is distributed along its
inner surface (e.g., inner surface 2602 of winding 2600). As a result,
although AC current will be most densely distributed near the inner
surface in order to minimize inductance, a significant portion of the
winding's cross-sectional area will still conduct such AC current because
a significant portion of the winding's cross-sectional area is
predominately distributed along the inner surface. Accordingly, the
configuration of the windings in coupled inductor 2400 helps reduce the
winding's RAC. The configuration of the windings may be contrasted
to that of winding 2200 of FIG. 22 where inductive effects may cause AC
current to be confined to a relatively small portion of winding 2200's
cross-sectional area. For example, an embodiment of winding 2600 having a
width 2608 of 3.0 millimeters and a thickness 2604 of 0.5 millimeters may
have a value of RAC that is approximately 8 times less than an
embodiment of winding 2200 having a width 2206 of 2.2 millimeters and a
thickness 2302 of 0.5 millimeters.

[0182] Additionally, as discussed above, each winding of coupled inductor
2400 may have a width that is greater than the winding's thickness.
Accordingly, such embodiments of windings of coupled inductor 2400 do not
have a completely symmetrical cross section. Such configuration of the
windings results in a larger portion of their cross-sectional area being
close to a surface of the winding. For example, the configuration of
winding 2600 results in a relatively large portion of its cross-sectional
area being relatively close to surfaces 2602 or 2606. Accordingly, the
configuration of the windings of coupled inductor 2400 helps reduce the
impact of the skin effect on the windings' current conduction, thereby
helping reduce their RAC.

[0183] Additionally, in some embodiments of coupled inductor 2400, the
windings span essentially the entire width 2402 of legs 2404.
Accordingly, the windings of coupled inductor 2400 may be relatively
wide, and therefore have a relative low RDC. Furthermore, the
configuration of coupled inductor 2400 and its windings may allow
embodiments of its windings to be shorter and thereby have a lower
RDC than windings of prior art coupled inductors.

[0184]FIG. 44 is a top plan view of one M-phase coupled inductor 4400,
where M is an integer greater than one. Coupled inductor 4400 may, for
example, serve as inductor 28 of FIG. 1. Although coupled inductor 4400
is illustrated in FIG. 44 as having two phases, some embodiments of
inductor 4400 have greater than two phases.

[0185] Coupled inductor 4400 includes a magnetic core including end
magnetic elements 4402 and 4404 and M rectangular legs 4406 disposed
between end magnetic elements 4402 and 4404. Legs 4406 connect end
magnetic elements 4402 and 4404, and each of legs 4406 has an outer
surface including a top surface 4408 (e.g., a planar surface) and a
bottom surface (e.g., a planar surface), which is not visible in the top
plan view of FIG. 44. The magnetic core of coupled inductor 4400 is
formed, for example, of a ferrite material, a powdered iron material, or
a Kool-μ® material. Although FIG. 44 illustrates end magnetic
elements 4402 and 4404 as well as legs 4406 as being discrete elements,
two or more of the elements may be combined. Furthermore, at least one of
end magnetic elements 4402 and 4404 as well as legs 4406 may be divided.

[0186] Coupled inductor 4400 further includes M windings 4410, which are
magnetically coupled together. Windings 4410, for example, have a
substantially rectangular cross section. FIG. 45 is a bottom perspective
view of an embodiment of winding 4410 before being wound about a leg 4406
of coupled inductor 4400. Winding 4410 has an inner surface 4502, a
thickness 4504 orthogonal to inner surface 4502, a width 4506, a length
4508, a center axis 4512 parallel to the winding's longest dimension or
length 4508, and a cross section 4510. Width 4506 is greater than
thickness 4504--such feature helps lower RAC as discussed below.

[0187] Each winding 4410 is wound at least partially about a respective
leg 4406 such that inner surface 4502 of winding 4410 faces the outer
surface of the leg. Furthermore, each winding 4410 diagonally crosses top
surface 4408 of its respective leg. Although each winding 4410 is
illustrated in FIG. 44 as forming a single turn, other embodiments of
windings 4410 may form multiple turns.

[0188] Each winding 4410 may form a first solder tab 4412 and a second
solder tab 4414 at respective ends of the winding. Solder tabs 4412 and
4414 are disposed along the bottom of coupled inductor 4400; however,
their outline is denoted by dashed lines in FIG. 44. Each first solder
tab 4412 diagonally crosses a portion of its respective leg's bottom
surface (e.g., planar surface) to extend under end magnetic element 4402.
Similarly, each second solder tab 4414 diagonally crosses a portion of
its respective leg's bottom surface (e.g., planar surface) to extend
under end magnetic element 4404. Solder tabs 4412 and 4414 are, for
example, integral with winding 4410 as illustrated in FIG. 44.

[0189]FIG. 46 is a top plan view of one PCB layout 4600 for embodiments
of coupled inductor 4400. Layout 4600 is illustrated as supporting a two
phase embodiment of coupled inductor 4400; however, layout 4600 can be
extended to support more than two phases.

[0190] Layout 4600 includes pads 4602 for connecting solder tabs 4412 of
windings 4410 to respective switching nodes. Each pad 4602 is connected
to a respective switching node shape 4604. Layout 4600 further includes
pads 4606 for connecting solder tabs 4414 to a common output node. Each
pad 4606 is connected to a common output shape 4608. Layout 4600
advantageously permits pads 4602 and 4606 as well as shapes 4604 and 4608
to be relatively large. Furthermore, layout 4600 permits pads 4602 to be
disposed close to switching circuitry and pads 4606 to be disposed close
to output circuitry.

[0191] As discussed above, each winding 4410 of coupled inductor 4400 is
at least partially wound about a respective leg 4406 such that each
winding's inner surface 4502 faces the outer surface of the respective
leg. Accordingly, the inner surface 4502 of winding 4410 forms the
smallest loop within the winding. However, as noted above, each winding's
width 4506 is greater than the winding's thickness 4504. Therefore, each
winding is configured such that a large portion of its cross-sectional
area is predominately distributed along its inner surface 4502. As a
result, although AC current will be most densely distributed near inner
surface 4502 in order to minimize inductance, a significant portion of
the cross-sectional area of winding 4410 will still conduct such AC
current because a large portion of the winding's cross-sectional area is
predominately distributed along inner surface 4502. Accordingly, the
configuration of the windings in coupled inductor 4400 helps reduce
RAC.

[0192] Additionally, as discussed above, embodiments of the windings of
coupled inductor 4400 do not have a completely symmetrical cross section
because their width 4506 is greater than their thickness 4504. Such
configuration of winding 4410 results in a larger portion of its
cross-sectional area being close to a surface of the winding, thereby
helping reduce the impact of the skin effect on the winding's current
conduction, in turn helping reduce its RAC.

[0193] Furthermore, the fact that each winding 4410 diagonally crosses top
surface 4408 of its respective leg and solder tabs 4412 and 4414
diagonally cross a portion of their respective leg's bottom surface helps
reduce length 4508 of each winding 4410. Such reduction in length is
advantageous because it helps reduce RAC and RDC of winding
4410.

[0194]FIG. 47 is a top plan view of one M-phase coupled inductor 4700,
where M is an integer greater than one. Inductor 4700 may, for example,
serve as inductor 28 of FIG. 1. Although coupled inductor 4700 is
illustrated in FIG. 47 as having two phases, some embodiments of coupled
inductor 4700 have greater than two phases.

[0195] Coupled inductor 4700 includes a magnetic core including a first
end magnetic element 4702 and a second end magnetic element 4704. First
end magnetic element 4702 has a center axis 4706 parallel to its longest
dimension, and second end magnetic element 4704 has a center axis 4708
parallel to its longest dimension. Second end magnetic element 4704 is,
for example, disposed such that its center axis 4708 is parallel to
center axis 4706 of first end magnetic element 4702.

[0196] The magnetic core of coupled inductor 4700 further includes M legs
4710 disposed between first and second end magnetic elements 4702 and
4704. Each leg 4710 forms at least two turns. For example, legs 4710 are
illustrated in FIG. 47 as each forming two turns where each turn is about
ninety degrees. Legs 4710 connect first and second end magnetic elements
4702 and 4704, and each leg has a winding section 4712 that a respective
winding is wound at least partially about. Top surfaces of windings
sections 4712 are designated by crosshatched shading in FIG. 47. Each
winding section 4712 has a center axis 4714 that is, for example,
parallel to center axes 4706 and 4708 of first and second end magnetic
elements 4702 and 4704, respectively. Each winding section 4712 has an
outer surface. Winding sections 4712 have, for example, a rectangular
shape. The magnetic core of coupled inductor 4700 is formed, for example,
of a ferrite material, a powdered iron material, or a Kool-μ®
material. Although FIG. 47 illustrates first end magnetic element 4702,
second end magnetic element 4704, and legs 4710 as being discrete
elements, two or more of these elements may be combined. Furthermore, one
or more of these elements may be divided.

[0198] Each winding 4800 is wound at least partially about the winding
section 4712 of a respective leg 4710 such that inner surface 4802 of
winding 4800 faces the outer surface of the winding section 4712.
Furthermore, the center axis 4812 of each winding 4800 is, for example,
about perpendicular to center axes 4706 and 4708 of first and second end
magnetic elements 4702 and 4704. Winding 4800 may form a single turn or a
plurality of turns.

[0199] Each winding 4800 may form a solder tab (not shown in FIG. 48) at
each end of the winding. Such solder tabs may be integral with winding
4800. Each solder tab may extend along a bottom surface (e.g., a planar
surface) of one of first end magnetic element 4702 and second end
magnetic element 4704.

[0200]FIG. 49 is a side perspective view of one winding 4800(1), which is
an embodiment of winding 4800. Winding 4800(1) is illustrated in FIG. 49
as having the shape it would have after being partially wound about a
respective winding section 4712 having a rectangular shape. Winding
4800(1) includes inner surface 4802(1) and an opposite outer surface
4902(1). When winding 4800(1) is wound about a respective winding section
4712, inner surface 4802(1) faces the winding section's outer surface.
Also shown in FIG. 49 are first solder tab 4904(1) and second solder tab
4906(1). Solder tabs 4904(1) and 4906(1) are, for example, integral with
winding 4800(1) as illustrated in FIG. 49.

[0201]FIG. 50 is a top plan view of one embodiment of coupled inductor
4700(1) including M windings 4800(1) of FIG. 49. Although coupled
inductor 4700(1) is illustrated in FIG. 50 as having two phases, coupled
inductor 4700(1) may have more than two phases. Visible portions of
windings 4800(1) are shown with cross shading in FIG. 50. The dashed
lines indicate the outlines of first solder tabs 4904(1) extending under
first end magnetic element 4702(1) and second solder tabs 4906(1)
extending under second end magnetic element 4704(1).

[0202]FIG. 51 is a top plan view of one layout 5100 for embodiments of
coupled inductor 4700. Layout 5100 is illustrated as supporting a two
phase embodiment of coupled inductor 4700; however, layout 5100 can be
extended to support more than two phases.

[0203] Layout 5100 includes pads 5102 for connecting solder tabs (e.g.,
first solder tab 4904(1) of winding 4800(1), FIG. 49) of winding 4800 to
respective switching nodes. Each pad 5102 is connected to a respective
switching node shape 5104. Layout 5100 further includes pads 5106 for
connecting solder tabs (e.g., second solder tab 4906(1) of winding
4800(1), FIG. 49) to a common output node. Each pad 5106 is connected to
a common output shape 5108. Layout 5100 advantageously permits pads 5102
and 5106 as well as shapes 5104 and 5108 to be relatively large.
Furthermore, layout 5100 permits pads 5102 to be disposed close to
switching circuitry and pads 5106 to be disposed close to output
circuitry.

[0204] As discussed above, each winding 4800 of coupled inductor 4700 is
at least partially wound about the winding section of a respective leg
4710 such that each winding's inner surface 4802 is adjacent to the
winding sections' outer surface. Accordingly, the inner surface 4802 of
the winding 4800 forms the smallest loop within the winding. However, as
noted above, each winding's width 4806 may be greater than the winding's
thickness 4804. In such case, each winding is configured such that a
large portion of its cross-sectional area is distributed along its inner
surface 4802. As a result, although AC current will be most densely
distributed near inner surface 4802 in order to minimize inductance, a
significant portion of the winding's cross-sectional area will still
conduct such AC current because a large portion of the winding's
cross-sectional area is predominately distributed along inner surface
4802. Accordingly, the configuration of the windings 4800 in coupled
inductor 4700 helps reduce RAC.

[0205] Additionally, as discussed above, embodiments of windings 4800 of
coupled inductor 4700 do not have a completely symmetrical cross section
because their width 4806 is greater than their thickness 4804. Such
configuration of winding 4800 results in a larger portion of its
cross-sectional area being close to a surface of the winding, thereby
helping reduce the impact of the skin effect on the winding's current
conduction, in turn helping reduce its RAC.

[0206] A coupled inductor has a magnetizing inductance, and each winding
of the coupled inductor has a respective leakage inductance. In some
applications of coupled inductors (e.g., coupled inductor 2400, 4400,
4700), such as in DC-to-DC converter applications, the leakage inductance
values may be critical. For example, leakage inductance values may
control the magnitude of the peak to peak ripple current flowing in the
windings as well as the DC-to-DC converter's transient response.
Accordingly, it may be desirable to control a coupled inductor's
windings' leakage inductance values.

[0207] In coupled inductors such as coupled inductor 2400, 4400, or 4700,
the leakage inductance values may be smaller than desired due to the
windings being disposed close to one another. In order to control or
increase the leakage inductance values, additional paths may be created
for magnetic flux to flow through the core. Alternately or in addition,
existing leakage flux conductance paths may be exaggerated.

[0208] For example, FIG. 52 is a top plan view of a magnetic core 5200,
and FIG. 53 is an exploded top plan view of magnetic core 5200. Magnetic
core 5200, which is an embodiment of the magnetic core of coupled
inductor 2400, includes end magnetic elements 5202 and 5204 as well as
legs 5206. Upward pointing arrows 5208 represent magnetic flux flowing
through legs 5206. Magnetic core 5200 could have two phases or more than
three phases.

[0209] In order to increase the leakage inductance values of a coupled
inductor formed from magnetic core 5200, magnetic protrusions or
extrusions may be added to exaggerate paths for leakage flux. For
example, FIG. 54 is a top plan view of magnetic core 5200(1), which is an
embodiment of magnetic core 5200 including M+1 magnetic protrusions 5404
(only some of which are labeled for clarity). Protrusions 5404 exaggerate
the path of leakage flux 5406; thereby increasing the leakage inductance
values of windings wound around legs 5206(1).

[0210]FIG. 55 is an exploded view of magnetic core 5200(1). It should be
noted that protrusions 5404 may be integrally formed with end magnetic
element 5202(1); alternately, protrusions 5404 may be separate elements
affixed to end magnetic element 5202(1).

[0211]FIG. 56 schematically illustrates one multiphase DC-to-DC converter
5600, which is one example of an application of the coupled inductors
disclosed herein. DC-to-DC converter 5600, which is an embodiment of
system 10 of FIG. 1, includes M phases, where M is an integer greater
than one. Although DC-to-DC converter 5600 is illustrated in FIG. 56 as
having three phases, DC-to-DC converter 5600 could have two phases or
four or more phases.

[0212] DC-to-DC converter 5600 converts direct current power at input 5612
having a first voltage to direct current power at output 5614 having a
second voltage. Direct current input power source 5610 is connected to
input 5612 to power DC-to-DC converter 5600, and DC-to-DC converter 5600
powers load 5616 connected to output 5614.

[0213] DC-to-DC converter 5600 includes M phase coupled inductor 5602. In
FIG. 56, coupled inductor 5602 is shown as including an inductor for each
of the M phases of DC-to-DC converter 5600. However, DC-to-DC converter
5600 could have a plurality of coupled inductors, where each coupled
inductor supports fewer than all M of the phases. For example, if
DC-to-DC converter 5600 had four phases, the DC-to-DC converter could
include two coupled inductors, where each coupled inductor supports two
phases.

[0214] Coupled inductor 5602 includes core 5604 and M windings 5606. Each
winding 5606 has a first terminal 5618 (e.g., in the form of a first
solder tab) and a second terminal 5620 (e.g., in the form of a second
solder tab). Coupled inductor 5602 may be an embodiment of coupled
inductor 2400 with windings 5606 being embodiments of windings 2600,
3400, 3800, or 4200. Alternately, coupled inductor 5602 may be an
embodiment of coupled inductor 4400, 4700, or 5700.

[0215] DC-to-DC converter 5600 further includes M switching subsystems
5608, where each switching subsystem 5608 couples a first terminal of a
respective winding of coupled inductor 5602 to input 5612. For example,
switching subsystem 5608(2) couples first terminal 5618(2) of respective
winding 5606(2) to input 5612. An output filter 5622 is coupled to the
second terminal 5620 of each winding 5606. Output filter 5622, for
example, includes a capacitor coupling output 5614 to ground. Switching
subsystems 5608, which for example include a high side and a low side
switch, selectively energize and de-energize respective windings 5606 to
control the voltage on output node 5614.

[0216] As discussed above, use of windings having rectangular cross
section promotes low winding AC resistance. However, use of windings
having circular or square cross section promotes short magnetic flux path
around the windings, and short flux path in turn promotes low magnetic
core losses. Additionally, use of circular or square cross section
windings also promotes small magnetic core volume. Accordingly, certain
embodiments of the coupled inductors disclosed herein have windings with
square, substantially square, or circular cross sections. "Substantially
square" in the context of this document means that winding width is
within 85% to 115% of winding thickness.

[0217] For example, FIG. 57 shows a perspective view of a coupled inductor
5700, which is similar to coupled inductor 2400(2) (FIG. 29), but
includes windings having square, as opposed to rectangular, cross
section. Coupled inductor 5700 includes a magnetic core 5702 including
end magnetic elements 5704, 5706 and N legs 5708 connecting end magnetic
elements 5704, 5706. N is an integer greater than one, and in the FIG. 57
embodiment, N is two. Coupled inductor 5700 further includes N windings
5710, each wound around a respective one of the N legs 5708. Although
windings 5710 have a square cross section in the FIG. 57 embodiment,
windings 5710 have a circular cross section in alternate embodiments.

[0218]FIG. 58 shows a cross section of winding 5710(1) taken along line
A-A of FIG. 57. Winding 5710(1) has a width 5802 and a thickness 5804.
Width 5802 and thickness 5804 are the same since winding 5710(1) has a
square cross section 5806. However, in alternate embodiments, windings
5710 have only a substantially square cross section 5806, such that width
5802 can range from 85% to 115% of thickness 5804.

[0219] Use of windings having square cross section may also simplify
winding formation since winding width and thickness are the same, thereby
promoting efficient use of winding material (e.g., copper). For example,
rectangular cross section windings with large cross section aspect ratios
are typically manufactured by stamping/cutting metallic foil on a bobbin,
resulting in waste of some of the metallic foil. Square cross section
windings, in contrast, can typically be cut to desired length on a bobbin
without winding material waste. Furthermore, it is often significantly
easier to bend square cross section windings along multiple axes and/or
in different directions than rectangular cross section windings with
large cross section aspect ratios.

[0220] While some inductor embodiments disclosed herein include two-phase
coupling, such as those shown in FIGS. 2-5, it is not intended that
inductor coupling should be limited to two-phases. For example, a coupled
inductor with two windings would function as a two-phase coupled inductor
with good coupling, but coupling additional inductors together may
advantageously increase the number of phases as a matter of design
choice. Integration of multiple inductors that results in increased
phases may achieve current ripple reduction of a power unit coupled
thereto; examples of such are shown in FIGS. 6-8, 10, and 17. Coupling
two or more two-phase inductor structures together to create a scalable
M-phase coupled inductor may achieve an increased number of phases of an
inductor. The windings of such an M-phase coupled inductor may be wound
through the passageways and about the core such as those shown in FIGS.
6-8, 10, and 17.

[0221] Since certain changes may be made in the above methods and systems
without departing from the scope hereof, one intention is that all matter
contained in the above description or shown in the accompanying drawings
be interpreted as illustrative and not in a limiting sense. By way of
example, those skilled in the art should appreciate that items as shown
in the embodiments may be constructed, connected, arranged, and/or
combined in other formats without departing from the scope of the
invention. Another intention includes an understanding that the following
claims are to cover generic and specific features of the invention
described herein, and all statements of the scope of the invention which,
as a matter of language, might be said to fall there between.